How could it be her upbringing? She chose to be a summoner against her 'family's' wishes. I doubt her father would have wanted her following his path either.

Yuna's problem was she was an idiot, and noone wanted to stand up to her because she was giving her life for Spira.
Twice in the game her and Auron's choices could have destroyed the world yet they were chosen as leaders. Not difficult choices either, they bot went out of their way to be stupid.
Plus Yuna was close minded. Her opinion on everything was "I will defeat Sin". That's it.