Quote Originally Posted by AssertnFailure
Reason #1, 2, and 3 are not enough. Why? Because I don't believe it's the act of killing your best friend that gets you the mangekyou, it's the act of consciously choosing to break your strongest bonds.
Perhaps it has nothing really to do with killing you're best friend/breaking bonds, but about hatred, specifically self-hatred. Itachi was always put on the proverbial pedestal. Always praised. As from what we're shown of Itachi, he never seemed to be simply Uchiha Itachi the boy. He was always regarded as the best of the clan. Maybe he had a mental breakdown because of how everyone had treated him, and Shisui just happened to be with him when he snapped, or caused him to with some idle chatter like "Itachi, you're the best." Since he would have been unable to do what he wanted as a normal person, he could have hated himself for becoming what he was.

Kakashi could logically have suffered from self-hatred, caused by his obvious guilt over Obito. He might have simply beat himself up emotionally over time enough, to the point where the latent Mangekyou awoke.

Itachi could have told Sasuke that he must kill his best friend, so that when it happens Sasuke would realize what had happened by his own hands, which might cause his to awaken as well. He might not necessarily care about awakening it, but just finding anyway possible to defeat Itachi. He most likely will, which will kill my theory, but Mangekyou doesn't seem to be an active priority for Sasuke.