What reasons does Jack list for changing his mind about being one of Auggie's "welcome buddies?"

From the text:

So here's why I changed my mind. It wasn't so I wouldn't have to hear Mom give me a whole lecture. And it wasn't to protect this August kid from Julian, who I knew would be a jerk about the whole thing. It was because when I heard Jamie talking about how he had run away from August going 'Ahhh,' I suddenly felt really bad. The thing is, there are always going to be kids like Julian who are jerks. But if a little kid like Jamie, who's usually a nice enough kid, can be that mean, then a kid like August doesn't stand a chance in middle school.