Hrrrrm, explaining it with their "special talents" and whatnot is really just an indirect way of saying "it's because they are what they are", since all those things only exist because they... are what they are.
Fluttershy is still a genuinely kind and caring person, but evidently, she
also bases her self-worth on the affection she's given by others. Given how shy she is, she evidently never quite got over her failure in flight school. Probably got made fun of, maybe even seriously bullied.
I guess I know the kind of frustration that made her snap. You're like, hey, new people, they seem so nice, I'm gonna make friends with them and it'll be so nice. And then you just kinda can't connect with them, but you want to show what a nice person you are and all, but the more you try to show it, the more you push them away, and bam. Tear-your-hair-out frustration.