I try to steal my friends' girls?

[Ineed]

The title doesn't really say it all. I'm not like this with ALL my friends, but sometimes if I see a friend talking to a girl online (twitter, facebook etc), I ultimately get jealous and have the urge to "steal" her from them) obviously id NEVER do anything if they were actually with them, but if they are just talking and the girl also gets on with my now and again, i'll make the effort to win her over. The first time i did this was 2 years ago and it was over skype. She eventually just really liked me and I just stopped liking her once I won her over. The second was an older girl and it was over facebook. I saw my friend flirt with her and I just subconsciously thought (I need to win her over) I ended up doing that but then I also stopped liking her! is there something wrong with me or a reason behind this?
they're not exactly friends anymore, I just knew them and used to talk to them. If they were a proper friend id feel very guilty. its still a douchy thing to do, but i really liked them...but then the feeling just went. thats why im so confused