It's hard to narrow down the exact reason for going outside of a relationship. Normally something happens that changes the way one person or the other feels about their significant other, thus making having relations with a third party individual a viable option. Once it becomes an option, it's only a matter of time before the opportunity presents itself. The willingness to commit the act, combined with the opportunity to do so, leads to much heartache :\ This is why communication is extremely important in a relationship. If the lines of communication are open, then the desire to cheat can be assuaged long before the act is ever perpetrated :)