It seems to be a case of "can't demonstrate what you haven't been shown".

I could see the writers going down the "if you are alone/the strongest nobody can hurt you" path, but I honestly don't like that take on Garble.

The way I see him he's cocky and actually loves to fight. It gives him a rush and an ego boost when he wins. Fighting could be a bonding exercise that is completely alien to ponies and would seem counterproductive from their perspective. It isn't like the dragons would actively realize they are becoming friends either, they'd just know they enjoy engaging in certain activities with others.

It has been established that having a competitive nature is inherent in dragons. Garble wouldn't be any different. Since he seems to have fought himself into a respectable position, he just doesn't take kindly to being defied and embarrassed by someone as weak as Spike, who hasn't put in the work to act like he has the same worth.

This is where I think the key difference is between Garble and Spike: Garble has been taught that respect is earned, whereas Spike has been taught that respecting others is the default. One believes that value is something you do not have unless you prove it and you cannot expect to be respected before then, the other believes that you are valuable and are entitled to respect unless you prove to be undeserving of it.