Originally Posted by Donut
I hope we get some reasoning with why he left the Mansion again. At the end of X3 he seemed happy there
There was a deleted scene which would have taken place after that:
I guess with Jean and the Professor dead, and Rogue no longer a mutant, he felt there was no reason to stay? Also, the realization of Jean's death may have taken some time to sink in (at which point, he might have started drinking heavily; become unsociable, and his teaching position would have been untenable).