1. cheating (like having an affair with her friend, or sleeping with her sister, or fooling around with every girl he meets)
2. too clingy (he wants the girl to be with him 24/7)
3. too controlling and over jealousy
4. girls meet new guys...
5. the guy cant live up to the expectations of the girl
6. lying (about everything, which leads to cheating)
7. family does not like the guy (and will disown the girl if she continued dating him)
8. they keep on fighting
9. he chooses to spend time with his friends rather spending time with the girl
10. just dating the girl coz of social status ( the girl is the head cheeleader and the guy is the football captain, or the girl is the prettiest girl and the guy is the cutest guy, or the girl is the coolest girl and the guy just want to be with the "cool crowd")
try one of these reasons, these reasons are true enough and can happen to anyone anywrhere anytime...