Jack's one of those high school students who feel as if they have to hang out with the popular kids in order to fit in. No one wants to feel like an oddball and Jack's no different in this regard. Hanging out with a popular group makes him feel accepted, gives him a sense of identity.
And yet Jack really doesn't enjoy hanging out with the popular kids, especially Julian, who's a thoroughly nasty and unpleasant individual. He spends time with these kids because he feels he has to, not because he wants to.
At first, Jack doesn't much care for spending time with Auggie, either. Like most of the kids at Beecher Middle School, he thinks of the new boy as some kind of freak. That he spends any time with him at all is due to the fact that he's been chosen by the Principal, Mr. Tushman, to be one of Auggie's welcoming buddies. But initially, Jack isn't that much of a buddy to Auggie and tells the other kids that he'd kill himself if he looked him.
Over time, however, Jack's attitude changes dramatically. He discovers that he actually enjoys hanging out with Auggie, which is more than he can say about spending time with the popular kids.