Most Helpful Guy
First, this isn't exclusive to guys. More appropriate question is why one would cheat another in a relationship, whatever the gender it is.
There are so many reasons that there is no one-fit-all answer. Most commonly observed would be:
1. One grows bored of the relationship. When being together for long, some people find it difficult to sustain it for any longer. This trait, no one could predict it when entering a relationship, but it might lead to this eventually. Some work it out with their partner, some choose the easier and degrading option to cheat. As a new partner, would bring back the excitement that's lacking in the current.
2. Inadequacy of certain elements in the relationship. When one attaches importance to certain elements such as materialistic possessions, traveling, sex life and similar passions and the other fails to meet such expectations, it leads to immense disappointment and frustration. Some choose to accept it and live with it. Some seek a way to vent, which leads to cheating in certain cases.
3. They were never in a serious relationship in the first place. One just manipulates the other to believe so. Then, cheating is the end result.
There are so many other cases. Whatever be the case, it's not a right thing to do and is degrading at many levels.