I'd suppose the majority of the players keeps coming back to it because
a) there's always new content. mappers keep creating new stuff every day and there will always be interesting music to play for all tastes b) the nature of the game allows for playing it whenever you're bored or dont have a lot of time to do anything big and just want to pass the time (think League of Legends queue) c) it's competitive. this only applies to a small margin of players but there's the people that have the time and motivation to keep playing to get better and beat others. it's a sport to them. d) the community, while having its issues, is actually pretty kind. You can spend hours on the forums or the ingame chat just talking to other people
I did have a wrong attitude before. If i didn't FC, i didn't conplete the map. Then i played bit too hard maps for me for like month. Then troed those maps i wanted to FC. And i did. I gpt huge amount of pp in a month. As ypu can see from my top scores. Then i quit for a wek and now i can't do anything. So should i now play bit more difficult maps to keep interest on?
For me, I set my priorities straight - when it comes to music and rhythm games, I play them primary one reason - to listen to and discover new music. So whatever I do in this game has to be related to that goal.
The biggest mistake aspiring players make in this game is to set too unrealistic goals - attempt to make it to the top of the rankings by playing Insane/Expert difficulty maps (and only those maps) and farm for PP. It's called the "I want to be as good a Cookiezi" approach. But from that approach, only two outcomes come out of it.
The first outcome is that the player is actually good enough to make it to the top of the rankings. That's extremely rare.
The second outcome (and the one that happens 99% of the time) is that players, when PP farming, hit a complete wall, don't understand why they're no longer gaining any PP, then come to the (false) realization that this game is too hard for them and thus it's just not worth their effort and time to actually put in the work to be good, and then quit.
What players who fall under the second outcome do not realize is that there's more to osu! then just playing osu!. First, by playing just the Insane/Expert difficulties, you miss out on half of the beatmaps you can potentially play. That narrows your music library. Second, there are three other game modes (and hopefully a fourth by the time the big new update comes out) that players can try. Third, you can create your own beatmap and also customize your own skin to enhance your gaming experience. And that's not to mention the Multiplayer aspect of the game, which players play mostly to have fun and meet new people. Many players do not realize this. They just see this game as "Oh, that looks cool, let me try it... oh, this is too freaking hard, it's not worth it" and they don't try to understand the true beauty behind this game.
I guess that's why most players quit and I'm still here.
It depends what you play for and what you want to get out of it. Some people play to listen to the music, some play to farm pp, some play to set good scores. Personally I play to improve and to be good at the game. A lot of my time spent on osu is training things that I am specifically weak at. I like being good at things that I invest a lot of time into, and I want to have to invest a lot of time to be good for it to be worth while. There is nothing satisfying about mastering a game that takes 2 hours to master. However a game that takes years, now thats cool.
First and most importantly, if you are not much into music to begin with I can understand this won't keep you entertained. Another thing is that you can combine this passion for music with a very common competitive spirit when you play for rank and compete with friends and others. The game also is very diversified since there are tons of tracks and maps to play.
I think this is basically why people stay interested in it.