A person can indeed cheat on their lover for no good reason.
I have been in a relationship for several years. My girlfriend is absolutely wonderful. She is kind, loving, and empathetic. She actually wants to get intimate more often than I do. However, when she is not around, there is still something inside me that craves the adventure of a random, no-strings-attached hook-up with some horny girl or woman.
Perhaps it is the teen libido still inside me, but I think it's more from simply wanting some excitement. While I do find my girlfriend sexually attractive, there is a part of me that is attracted to the idea of meaningless sex, and with my girlfriend, no such thing is possible. I don't cheat often, but I also don't go out of my way to do so. It is more like an itch I scratch every once in a while. I cheat on her about once every year, and when I select a person it's usually only once, and usually involves the lowered inhibitions brought on by a social night of drinking. It is also usually not planned, which for me simply adds to the excitement.
Some say it makes me a scumbag. Others say it makes me a callous, insensitive monster. I can't argue with either perspective, because mine is biased, but in my own opinion I don't have a problem with what I've done. It makes me and another person feel good, and as long as my lover never finds out, it doesn't hurt her, which seems to mean it isn't wrong. Maybe I lack a requisite guilt impulse, but on a purely practical level, a lie that doesn't cause someone harm is not a harmful lie, and the only characteristic of something that defines it as bad, to me, is whether it causes harm to someone. It has also not damaged my relationship, as we are still together and I have no intention of leaving her. I love her and care about her happiness.
Please, feel free to comment me to agree, disagree, or share.