Board Thread:Naruto Discussions/@comment-24098614-20140910222248/@comment-24098614-20150108093334

The Second Hokage mentioned that this is the greatest weakness that the Uchiha have because they let their emotions get a hold of them, and they can't let go, and they get obsessed with it, and it relates to the Sharingan because the more negative feelings you have the stronger that bloodline gets especially hatred and anger those two feelings give the Sharingan the greatest power.