Girls aren't always the ones to break up with guys and it's really hard to generically answer why relationships end. What I can do is list the top 5 reasons I've broken up with guys.
1) They were too clingy
2) They cheated or were acting like they were going to cheat
3) They tried to control me
4) They kept saying I was "out of their league" - HUGE turn off (obviously if I got with them I didn't think I was out of their league but once they started saying that over and over again I started thinking I was)
5)They tried to get me pregnant
If you're not doing any of those things then sometimes relationships just don't work because personalities clash or for some other reason.