Okay, an odd thought.
I've seen a lot of people assume that, because Fluttershy spends a lot of time on the ground and has such talent with animals, she
must have some earth pony ancestry. Now, I'm not a fan of the view that a pony's entire personality is determined by their genetics... But if we were to entertain that view, why not go for a less obvious application of it? Namely:

Rainbow Dash is only half pegasus.

Rightly or wrongly, she thinks the other pegasi judge her for her earth pony and/or unicorn genes. Her competitive nature and her bragging are overcompensation: she's trying to prove—to all the doubters, and to herself in particular—that her ancestry doesn't make her any less of a pegasus.
I don't particularly like that train of thought—and I honestly hope it
isn't canon—but I found it interesting all the same.
edited 1st Oct '12 9:19:14 PM by MetaFour
The portable Venus of occupied space. Grace is a maze of amazing taste.