Aniston comes across as a very needy women who demands a lot of personal attention from Brad. IMO, Brad is the type of guy that doesn't want to feel strapped down to some kind of a "one girl" committment relationship. I feel that Aniston really feels insecure about the kind of publicity and attention Brad gets. Whereas Jolie, she is beautiful, much more beautiful inside and out than Aniston. Brad feels that Jolie has more confidence and is more secure with herself therefore, he feels that Jolie is the "one" and only.
This is just my opinion.