why does ackley hate stradlater?

Ackley didn't like Stradlater because he embarrassed him. He told him to brush his teeth once in awhile.

"The reason you're sore at Stradlater is because he said that stuff about brushing your teeth once in a while. He didn't mean to insult you, for cryin' out loud. He didn't say it right or anything, but he didn't mean anything insulting. All he meant was you'd look better and feel better if you sort of brushed your teeth once in a while."