Ah, my bad misunderstanding - so you were talking about her upbringing.
Well, Madoka's family IS kind and loving. And Madoka hasn't ever been hinted to experience any direct trouble outside home either. She's always had good examples to look up to (we'll skip the "can't wait to get drunk with you, Mom" part, OK?
), and there doesn't seem to have been anything to make her bitter and disillusioned about the ideals she had. Although the second factor may be less relevant - we don't see Madoka giving up on her personality even things DO go downhill. Although the emotional breakdown she undergoes is more than visible.