Most Helpful Guy
For the same reasons women do. They're ignored, taken for granted, talked about to friends, lied to, and in many situations their emotional trust is betrayed. Most of the time, it literally comes down to the fact men AND women have their emotional bank over drawn.
Women & men cheat usually because a guy, or girl (other than their SO) is giving them emotional support. After a while, women AND men emotionally bond to someone else and feel a desire to be with that other person. That doesn't make it right. BUT... the person cheated on can possibly be partly to blame for the relationship break down. It's far from always being one sided.
Most Helpful Girl
It's the similar reason for every cheater
1. Didn't get the lovin they needed/ attention/ whatnot
2. lack of self control/ puts themselves into situation/temptation / example: taking work out at cafe/ or spending time with someone else/confiding feelings etc
list goes on