What can cause a guy to leave a girl?

Cheating, abuse (usually emotional), a big lie, flirt with other guys too much(like daily bases), guy's health problem, girl too bossy, too control on the guy(no freedom), check on him every hour(msg,call etc. ), when the guy met new girl his girlfriend start to use the judging way to ask about that new girl etc. A lot of things

Usually its same as what can cause a girl to leave a guy.

Cheating, lying, abuse, whether it's physical/emotional (insulting the other person constantly would go under this category).

Desperation - Now this one will tend to make emotionally healthier people run for the hills.

An unhealthy level of jealousy, or the feeling that you need try to cause this unhealthy level of "jealousy or the love isn't true" stuff.

Trying to start fights. CONSTANTLY. If you're fighting every day for 5 weeks about little things, then the dating relationship either needs serious work, or it needs to be done and over with.

Those are the big reasons. Sometimes the breakup can be a small reason, sometimes big; sometimes it's a hint of bigger things that you know you won't tolerate down the line. Or sometimes it just falls apart for whatever reason. Maybe a huge move, or a huge life change.

I think I'd personally have trouble with a long distance relationship, for example.

There are millions of reasons, ranging from silly to serious, and it depends on the couple.

Cheating, emotional abuse, issues with sex, fighting, lying, controlling, personality, attitude, etc. All the common sense stuff.