Was Marty turning a bit on George?
Right before the incident when Marty does his dare, George squirts the water gun on Marty. Marty being the one that usually rough-houses everyone else seems to respect George in that moment in an "Alright, that was a good one, you got me" sort of way.
Things quickly derail after that, but I think given enough time, Marty might have grown to appreciate George as well.