
Originally Posted by
nivlakian
This is purely speculation:
Itachi claimed the requirement for Mangekyou was killing your best friend. I would wonder what about that action would trigger it, and I'd assume it would have to be the emotion associated with the act. In Sasuke, we've seen activation and tomoe upgrades come from life/death types of situations where emotions would probably be taken to an extreme. So, based on that, one might guess that Kakashi blames himself for Obito's death and convinced himself that he killed him or those emotions festering for a long period of time allowed him to trigger it with some additional amount of effort/strain since he is not an Uchiha.
Otherwise, there could be an alternate requirement that Kakashi discovered.
*shrug* They'll come up with something if they do ever decide to talk about it.