Brad Pitt grew up with the real people of the world. His parents are not part of the "Industry". His folks are salt of the earth Christians from Springfield Missouri. Brad grew up in the Ozarks and went to Kickapoo High School. His mom and dad are about as normal as any two people can be. His brother who is literally his best friend runs a Computer Store in Springfield. He is extremely close to his family, talks to them literally everyday. They are his grounding and his guidepost.
He knows what real marriages are, what real relationships are and that is what he has been seeking in his adult life. I believe his disapointment in his marraige with Jennifer Anniston started along time ago. When they got married they had set goals and her goals changed mid-stream. It doesnt make her a bad person , it just means she no longer wanted what he did, she wanted something different. He has always been very clear on what he wanted. He wanted a family and he believed Jennifer Aniston had the same goals when they married.
I think in his mind he can justify how he left his marriage because he felt that his wife had lied to him and decieved him for five years regarding children. I think that one thing had eaten such a hole in their marriage that he had emotionally left the marriage long before Angelina Jolie came into the picture.