Why is my friend flirting with me and showing interest when he already has a girlfriend?

-I know at the very least he thinks I'm an attractive friend, but I'm getting the vibe he's starting to like me because he's showing a lot of signs. He's not a player, and I'm definitely not the type of girl that looks like she's willing to do a side thing...so I'm wondering why he's initiated a "thing" between us. I mean...I would no doubt consider dating him if he were single, but that's not the case. I do want to talk to him about it at some point, but the right timing hasn't come around (and I'm probably much too shy to bring it up lol). Why do you think he's doing it (from a guy's perspective)?Hmm, that strange, its as if he wants to get in trouble.