I think part of what causes people to get bored with some stat-raising sims is the long wait for feedback, or the lack of gameplay.
Just as Anna pointed out that the boring parts are picking a weekly activity and confirming it over and over again. Here are some general ideas I've picked up and notes I've made from studying games like the Princess Maker series, True Love 95, and Papillon's own Cute Knight games.
1.
Feedback is key. Player's need to see the results of their actions and understand what they mean. This means if one job or activity raises some skills and lowers others, the player sees that happening as the activity or job takes place, usually by showing those stat bars on screen. A summary pop-up at the end of the week is great too, for instance: "Result of 1 Week of Activity X: Strength up 5, Refinement down 4, Stress up 30." Players should also be able to know what each job or activity does before they select it. Princess Maker 5 does this by telling you what a job or club will do before you put it on your planner, for instance: "Babysitting will greatly increase Sensitivity, but will decrease Charm some." It lets players make informed decisions.
2. This goes back to Number 1. Some randomness in stat gain is fine (maybe even preferable), but
stat gain still needs to be deterministic. Player's should NEVER not get a stat gain for an activity unless they have maxed out that skill. In the Princess Maker series you can do so poorly at a job that you don't get any money from it, but you still get the stat increases. In their school or lesson activities you can do so poorly for a day that you don't gain the stat increases, but you are told this - "Your daughter slacked off in classes today, no progress was made."
More importantly, the player knows WHY no progress was made. In Princess Maker it means your Stress was too high or your daughter was delinquent (caused, again, by having too much Stress). Stress is displayed on ever job and activity screen, and it is easy for a player to watch and see that high stress correlates to poor performance.
The Princess Maker series also rewards a player for carefully managing things so they get a full week of successes by giving a bonus to pay. Princess Maker 5 introduces random "very good" success to lessons that grant extra skill points. The player is told when this happens with a snappy graphic on screen.
There are also always a way to gain skill points without doing a job - be it activities, school lessons, or buying an item. This helps players make up ground in a skill if they decide to switch to it later, or let them raise opposes skills (like Strength and Intelligence) without decreasing the other.
3.
Rewards. Just as Papillon said, poor tradeoffs annoy people, and for good reason. If something is hard to obtain, it needs to be worth it. And you need to make sure that your stats correlate properly with each other. By that I mean, related activities and jobs should raise and improve (and decrease) related skills. So if a player is focused on raising their physical stats, all the physical jobs should help do that, and no job should ever, say, increase Strength but decrease Constitution.
Rewards go further than that though. Players like to see new things and be recognized for excelling or focusing in certain areas. In True Love 95, a player could receive a stat boosting visit from a spirit if they focused hard on one skill, or even open up a whole new romanceable character if they went above and beyond in increases certain stats. In Princess Maker 2 a god or goddess would visit and reward the player with stat bonuses for working hard on just a certain area of improvement. In both True Love and Princess Maker, these stat bonuses didn't just fill up the max meter for that skill faster, they actually counted ABOVE the max, so you could end up with 110% or more in a skill.
4.
Surprise and novelty. A big draw of the stat-raising sims is seeing what ending your stats or actions will give you. This is one reason these types of games have a huge number of endings - that's a lot of the fun. So players of these types of games like to see their choices and decisions awarded with different outcomes. That's almost the entire point of playing - what outcome will I get this time?
But surprise and novelty isn't just in regards to the endings. These games have a lot of fun surprises built into the game itself. Like the rewards of stat bonuses above, these things tell the player - "Wow! The game recognizes how hard I've been focusing on this one area! It's paying attention to me!" One of the things that keeps drawing players back besides the endings is that they are always discovering new things. In Princess Maker 2 you can discover bandits while adventuring, defeat them, and the townspeople will acknowledge your feat. You can meet different creatures depending on items you have, or marry the Devil himself if you meet him and your Sin is high enough, etc.
5.
Entertainment. Anna was right about how boring weekly activities can be if forced to do them over and over again. One way these types of games mitigate that is by having
sprite animations showing your character doing the job or activity, with changes for if you are screwing up. These can be very entertaining to watch. Princess Maker 5 has a variety of these for each activity - one for good, one for bad, one for very good, etc. Some have multiple animations for "good" since you are likely to see those a lot.
But that is just one form of entertainment. Another is variety. Generally a large selection of activities and jobs are available so the player can alternate between different things and keep from getting bored. For instance, there are usually multiple ways to raise a particular skill, so you don't have to repeat the same job or activity over and over gain just to raise a skill you are focusing on.
Another form of entertainment is differentiated gameplay. This means that the farther into the game the player gets and the more specialized and advanced in their skills they are, the more different the game becomes. For instance, in Princess Maker 2, if you focus on fighting, physical skills, and adventuring, you have the opportunity to get rewards for killing bandits, win fighting tournaments and receiving special items, fight duels on the street, and even fight the God of War himself. Compare that to another game where the player focuses on being charming and refined and starts getting special dresses made, dancing at balls, having suitors come calling, having meetings and conversations with royalty, etc. This means that the more new things or ways of playing a player tries, the more they discover.
6.
Events. The more of these you have, the better the game is likely to be received. These are both a reward for the player, as well as feedback. Events can pop up due to interesting choices the player has made with stats, or buying a unique item, or being somewhere at a certain time. In Princess Maker 2, the player can receive a new outfit if they are a Charming adventurer by being beautiful when they encounter a dragon, for instance. Festivals or contests or dates are another way to let player's test their skills or receive rewards and new plot by having certain stats. Did the player ace that end of month exam? Maybe the girl that tutored them the most will surprise them with a celebration date or party.
7.
RPG Elements. Combats, fights, exploration, weapons, armor, enemies, adventuring - these are all things that help games like Princess Maker or Cute Knight appeal to a broader demographic . For instance, as a man, I don't really play Princess Maker to make the daughter a pretty, pretty princess or see what cute boy she will marry. I play it to raise a little butt-kicking adventurer who slays monsters, finds treasure, drinks with the Devil, and then comes home to buy a new dress with her money. Or at least that's what I did at first, but thanks to all things I've mentioned above, I kept playing because it was a fun game, so eventually I did make her a princess, then a queen, a magician, a dozen other things.
If all the other numbered points I've made are followed, this element (adventuring and fighting) can be skipped and it will still be a fun game, but it certainly helps.
Finally, all these points are part of one big overarching theme - replayability. If your game doesn't vary except in the endings - for instance, if studying with all 3 girls feels the same and only the dates and endings are different, then replayability will be hurt, because players will fall into the same pattern of boredom that Anna complained about. They will say, "I want to end up with Girl A, so I will just choose her to tutor me and do the skill that adds an affection point for her." Then they will get bored if they have to do that 30 times over and over to get her ending. This is especially a danger with what you said that 100 Affection points are possible and a special ending is available for each girl if the player has all 100 Affection points. If only 100 points are available, and 100 points are needed for a special ending, that gameplay run for that special ending is going to get awfully boring.
And Papillon is right about the way to handle stats affecting relationships. You should never have a declaration of love in the game with a character unless you have already met the stat requirements and qualified for their ending. That is indeed just bad writing, not a symptom of dating-sims themselves. The important thing to remember when making one of these types of game is it is indeed a GAME. You can't write it as if it is a Visual Novel or a Kinetic Novel. You are giving the player choices and stats they are purposefully building. You have to let them drive the story, not force a pre-determined story on them. The stats have to be there for a reason, not just because you felt you needed the player to do something in-between story scenes. Otherwise you may as well just have the player make a dialogue choice about which girl he wants to go out with.
Anyone that thinks dating or raising-sims are boring are just playing the wrong ones - just try out some of the examples I've mentioned in this post - True Love 95, Princess Maker 2 (both in English), Princess Maker 5, etc. True Love 95 is an eroge game, but it has very deep storylines with all its characters - each girl's path is almost an entire Visual Novel in and of itself.
as you click buttons to plan out your weeks. There aren't many different scenes and it gets very repetitive. I think sims that do well are ones where there are a lot of unique scenes spread throughout the game so you don't spend more than 15 minutes at a time just clicking and seeing the same "Yay! My intelligence went up +10!" animation over and over and over. There should be other rewards for raising your stats, too, other than just clearing the minimum needed for an ending. In Machina Jewel, the first VN I started writing/programming, the stats the player raises are personality point stats and they affect how the heroine talks and responds to situations. It also helps that while the guys prefer certain personalities, as long as you spend time with them and whatnot, you can still have a relationship even if you, for example, go after the intelligent guy despite not focusing on the intelligence stat yourself.
How would the person even know you have those points, ha. All of my game's stats are built up because of spending time together like you mentioned.