Actually, the one I find more reasonable is the theory that Bloodraven (the Three-Eyed Crow) was the one who whispered to Aerys, making him go mad and setting into motion a chain of unfortunate events. Now, he is teaching Bran to be a greenseer, but telling him he can't communicate with people from the past...when he clearly can, at least to some extent. So some believe Bloodraven is telling him that to prevent him from communicating with the past and making the same mistakes he made...possibly because Bran can be more powerful than he is...