Culturally speaking, Why is Will Smith so rude to Brad Pitt?

alright yo listen i was there you can kinda see my head in the corner. but ya. anyways. will was all up in brads grill like man you dont know me why you gonna act like you know me. and brad was like man im just tryin to be your friend bro. you know man, like your brother. and will was like pshhh and brad said, hey will....f.u....i got spit on a little when he yelled that. score...