Advice from GUYS please....why do you do stuff like this???

so this guy in one of my cles flirted with me for about a month before asking me to hang out. we had a good time hanging out, and i let him know that i wanted to hang out again. after that, he started acting obnoxious, immature and not at all like the polite guy he was before. he started making gross comments, and acting like a guy would act around his rowdy buddies. i don't want to know how to get his interest back, i just want to know why so many guys do this?? it's just immature and weird.