Why does he do this?

this guy flirted with me lots then we got talking online and got on reli well. he eventually asked me out but i didnt get the message till nxt day then said if u still want then yes but he changed his mind. then he got with another girl. i try to ignore him but he keeps pestering me and wants to still b mates. i dont get why he is so bothered bout being mates when if he cared the slightest he wouldnt b wiv another girl.