I suppose the reasons for cheating could differ. Some people cheat because they get off on getting away with something. If that's the kind of person you're dealing with, run away. Some people cheat because they find someone else so incredibly attractive they can't help themselves.
Many of us are attracted to other people no matter how good the relationship we're in is. The better of us talk it out with our partners. I personally have an arrangement with my husband that allows us to have other partners. We don't consider this cheating because it is done without lies, with full knowledge and consent of all parties. This arrangement is certainly not for everyone, though, and it requires a lot of communication and self-examination.

I've only ever cheated when the relationship is so torched that I no longer feel like I'm in a relationship. I'm not proud of this, of course, especially the times when the other person disagreed with my relationship assessment. I should've just manned up and broke up with them. Nobody can keep me dating them against my will, regardless of what it felt at the time.
In a good relationship, sometimes people cheat because it's too good, and they don't feel they deserve it so they blow it. Sometimes it's to make sure that they are committed, as weird as it sounds. That's where you get the more stereotypical male response of "she meant nothing to me, baby", which is to say I f*cked her and you're better and now I know. A lot of people cheat because they have a disconnect between sex and love. Some people cheat just because they're not monogamous, and that's not very culturally acceptable and so they try and fail.

I've never cheated in a relationship, but I know I have thought about it... my reason being: I like sex a little more on the wild side... kinky kind of stuff... hot wax, restraints, etc... my current boyfriend isn't into that at all. I love him with all my heart, and we've had conversations about him trying the stuff, and he said he would (which, he hasn't yet) but he's really more of a 'vanilla' kind of guy. I have a guy friend who, not realising I was in a relationship said "Hey, you should come over and I can do *explicit content* and then maybe we can *more content* and to be honest, the only reason I didn't go was because I know that I am completely in love with my current boy friend and never want to hurt him. I also know that if I weren't in love with him, or if I were tired of the relationship, or if I just wasn't turned on by him any more, I would have been at my other friends house doing all matter of kinky things.

Something that would make me cheat was if I felt something was missing (which is possible even in very good relationships). That said, however, I have never cheated and don't see myself doing it in the near or distant future. I think it's much better to talk to the person you're currently with and see if you can resolve the issues there first. If you can't, then you end the relationship and move on.

rubus

i think he would have to cheat on me and even then it would be hard to do...

Earth~Goddess

Personally, I think there is NO excuse for cheating. If a person can't restrain themselves around other attractive people, then they owe it to their significant other to break it off so they don't hurt them more than is required. To me, there are 2 things a guy can do that deserve NO second chance - the first is cheating, the second is physical abuse. The first offense of either of these two actions will be the last because it will be over.