It happens all the time, and guys aren’t the only guilty ones. But why? Well, I think it comes down to a few reasons.
Remember when you were a little kid, and somebody had a toy that you didn’t have? Maybe it was an action figure, or a doll, and you had kind of forgotten about your action figures anyway, gotten over the excitement of them. But when your friend or sibling showed up with a version you didn’t have, it was killing you. More than anything YOU want to have that toy in your own hands.
But it’s also the thought of conquest – if someone can get the girl/guy who is already taken, it’s even more of a challenge, more of an achievement for them. It’s like the ultimate “hard to get” situation, and humans love the chase.
But most of the time I’d say people hit on their friends girlfriends/boyfriends because they’re jealous. They see someone that they already know is relationship material, and a part of them wishes they had that.
Either way, it doesn’t say much for your friend. Clearly they don’t care much about your happiness, and unless you’ve done something to make them want revenge, it’s probably just because they’re insecure. Lack of self confidence often ends up in a need to feel superior to the people around, instead of confronting issues of self esteem head on and being happy for your friends.
But it’s important to remember that this will probably happen no matter what, so the end result really depends on the strength of your relationship. If someone cheats, it can’t be blamed on the outside person; the person IN the relationship has to take responsibility for their actions, and admit that there’s a serious weakness in the relationship that needs to be confronted.