Board Thread:General Discussion/@comment-4310945-20131229044249/@comment-11051098-20140104194120

Yeah, he feared to die as a Shinigami, for at least 100 years or more. He wanted to become more Hollow-like.

I don't know if it's been confirmed exactly when Aizen stole Kisuke's Hôgyoku and fed it to his own. It could have been before the Hollowfication incident, or it could have been long after.