People. This is what happened.
Brad was a serial cheater. He had cheat on Jen before Angelina came along. I know of someone personally who slept with him and he cried and cried after. Apprently it was an ongoing problem with them. HE IS A DOG.
Then he cheated again, but with Angelina. This time, it was public. Brad and Jen packed it in, he moved on - which was also made convenient for him as Angelina is considered this this saintly goddess of good and sex, all wrapped into one. Also, it was a ready-made family for him, better than, say, leaving Jen for an extra he happened to screw on the set of Troy.
Anyone recall the TRASH tshirt he wore on Brad & Jen's farewell holiday together, just before announcing the split? He knows he was a sh*t, etc etc but he couldn't keep it in his pants and finally he gave his heart to someone else, BECAUSE HE WAS/IS AN UNFAITHFUL CAD , not because he was 'trading up' as it were.
My conjecture: Angelina has a dangerous edge (& the kids!) that he hopes will keep him both 1) in line and 2) in the spotlight.
Hollywood's a weird place, but sometimes these things are a bit too obvious to be comfortable.