1. Because they can! Their partner often "forgives them" and takes them back reinforcing that it is "okay" and really not that bad.
2. Because "boredom" sets in... The novelty wears off and they need a new fox to chase.
3. Because they are selfish and fulfill their own wants, needs and desires even if at the physical and emotional expense of another.
4. Because someone cheats with them! It takes two to cheat and many of the cheating partners know the person they are with is already taken. For many, it adds to the excitement factor to know that they are "stealing" someone away from their original lover.
5. Because they have learned it from their parents, movies, tv, magazines... They have watched either parent cheat on the other and lead to a divorce or disrupted home life and it is a common theme in many shows, movies and articles. It is acceptable to them.
6. Because there are so many opportunities available. College puts a ton of like-aged, like-minded, similarly-wired people together in a close knit community with few rules and a great deal of freedom and opportunity. This does not mirror real-life where meeting others is a much more difficult proposition.. Men figure if they lose the woman they are with, there are plenty more to replace her.
7. To break off a relationship they want to terminate. For some men, it is easier to be "caught" being interested in someone else, than to tell their current partner that they are no longer interested in them.
8. Because they are entering their sexual prime and cannot (or wish not to) control their desires to "test the open market."
9. Because men see a commitment as a loss of freedom, while women see it as a positive addition to their life.
10. To make sure that "they still have it!" Guys sometimes need reassurance that they are attractive to and desired by others.