why does ponyboy feel he and his gang dont belong with tim shepards gany and the brumley boys

Ponyboy believes they're different than the other gangs. They aren't simply out to rob and hurt people.

Ponyboy notes that the difference between their gang and the Shepard gang is that Tim Shepard uses "strict discipline;" there is a clear leader and they are organized, while Ponyboy's gang is "just buddies who stuck together."

I found the quote;

"He shouldn't be here." None of their gang should be there, because "We're greasers, but not hoods, and we don't belong with this bunch of future convicts."