Why are guys like this?!?

there is a guy tht I like and he likes me to, but he has a gf. we don't go to the same school, and so we barely see each other bcz he is always busy. he kissed me and stuff, tht one time tht i saw him. in the begning, when we first met, we were constantly talking to each other and stuff. but how come there are days where he is super flirty on msn and stuff and other days, he's like blahh. it gets me so annoyed when he is always busy and he's not always in the constant mood to me. and i dunno y he is taking so long to break up with his gf, when he likes me and he told me a while ago tht if she doesn't chnage her attitude, tht he would break up with her...but y is he taking so long to do it? and plus he shoudln't be kissing me and stuff while he is still dating her... wht should i do?