Board Thread:Theories and Speculation/@comment-28456867-20160525000548/@comment-4912646-20160526234843

Probably the same reason he unleashed the Nine-Tails on Konoha, took control of Kirigakure, participated in the Uchiha massacre, and went out of his way to ensure Sasuke was on his side. Not sure what that reason is, mind you; the series never told us why he did anything.