How can I get him to like me?! Please help!?

There's this guy that I really like, but he already has a girlfriend who's rich, pretty and outgoing. He always flirts with me in homeroom, first period and resource (he pokes me with his pencil a lot, tickles me, etc). Why does he flirt with me when he already has a girlfriend? If it's possible, how can I get him to like me?