There are a few things that have come up for me. I have inconsistent play time and part of it is just not feeling like it, but there are some real reasons too.
1. Sleep. In the past osu! has interfered with my sleep, or at least made me think that it was. It just makes me feel more "aware" at night even if I don't play before bed, but I still feel rested. It's very strange and it sometimes happens regardless of osu! or not.
2. Burnout/Tilt. In the past I've gotten tilted because I wasn't ranking up quickly enough, but I realized that not only am I not playing consistently enough to really expect huge gains, but the way I play is not super rewarded anyway. I don't farm I just go for sightreads (look at my A ranks) and try to improve my individual skills. Also, I suck at aim so I can't farm that well anyway until I work on that. Basically, rank doesn't equal improving and I can set my own goals. So I don't really stop because of that anymore.
3. Hand pain
This is the main one for me lately. Ever since late last year I had hand pain when I realized I could pass 6* with decent acc, and it was so much more fun than 5*. But I started overdoing it one day and just spamming the hell out of some long intense map and some touhou map to test my stamina and new abilities. But I just went way too hard at the end of the day and played through numbness like a dumby. I didn't have the self-control to play less so my hand just hurt so often I stopped playing altogether.
So yah that's three things that have made me quit before, but I think I've dealt with all of them and now that I'm getting into it again I am taking it really slow (30 min a day for a week) and doing hand exercises before I go into longer sessions.