1. He Already Knows You[Obviously]
There's something really nice about getting comfortable around each other — guys think so too. But with that familiarity, he loses some of his intense desire. What is always sexy? Mystery, and nothing is more mysterious than the unknown. And what's unknown? What other women look like naked.
2. His Buddy Kevin Did It
If one of your guy's friends screws around without getting caught, your guy may suddenly look at cheating in a whole new light. After all, men are driven to do incredibly stupid stuff if their friends encourage it or do it first — take, for instance, the Jackass movies, or dudes who wear their hair in that spiky, Growing Up Gotti way. So if his pals tell or show him that there are no consequences, he may forget all of the other reasons there are not to cheat.
3. His Boss Tore Him a New One
When a man's ego is ripped to shreds, the quickest way for him to pick up the pieces is by proving he can still attract, seduce, and have sex with any woman he wants...or, alternatively, with whatever chick is still hanging around the bar at closing time.
4. His Fantasy Football Obsession Doesn't Bug Her
If some other woman isn't aware of his annoying habits and hasn't been forced to endure them for a prolonged period of time, it's like they don't even exist. And seeking out that arrangement is much easier for some guys than actually altering their goofy behavior for their girlfriends.
5. A Hot Chick Just Walked Past Him
Yes, men know women check out dudes — but not in the same way men check out women. Guys immediately imagine ourselves having sex with anyone who's at all attractive, and that constant stimulation can be overwhelming. It's like if every good-looking guy you saw was also writing you a poem while saving injured kittens on his motorcycle.
6. He Can Rationalize It
Men love games. And games are all about how far you can bend the rules. So a lot of guys have very elastic definitions of what cheating is. Hence groping and kissing is fine if it's on a dance floor...and besides, being drunk makes pretty much anything okay...oh, and of course it goes without saying that strippers don't count...
7. He Realizes He's a Coward
Sometimes guys cheat not because they want to cheat, but because they want to end things altogether. He's thinking about the "talk" about the "relationship" that will probably become "emotional" — and who wants to deal with that stuff? So instead, he'll either pretend that he's not in a relationship when it suits him, or he'll behave so badly that he forces his girlfriend to end things for him instead.