Why do guys like to chase girls?

i like this guy a little and he is so crazy about me. he told me he loves me but i told him i dont love him. i ignore him phone calls, deleted my msn so he wn't be able to send messages, told him not to call me but still he does all kinda things just to talk to me. he said he is not doing all this for ....he said if there was someone else on ur place, i would have forgotton her, but he said i have touched his heart and he miss me a lot....he said he stay up all night just because he thinks i would go online......but i didn;t......and there was this guy i really liked before,,,i told him i like him then he didn't pay any attension to me like he used too......so why do guys do this?