Great topic. While upbringing does have a profound impact on you, it doesn't mold who you are. I believe Harry made the right choices because of who he was, James and Lilly's son. He was a nephew and treated horribly, but he wasn't a Dursley. He made the choice not to join the "cool crowd" when given the opportunity and that belief in self was what guided him throughout. When you have no one as Harry didn't, all you have is yourself and he formed a solid idea of who he was based on his treatment, how he wished he were treated and what he felt in his heart. I do believe some of his temper is rooted in his upbringing, which comes out throughout the series as does his feeling of alone. All of which is understandable.