May be some withdrawals from the fast life and he knows it. Back in his Pride days and initially in the UFC he was making a ton of money, smashing chicks left and right, and was living like a celebrity. Those days are gone now and maybe it isn't that he wish he didn't do it more as he doesn't know what to do with himself now that it is all just about over and it is hard to come to grips with it.
Or he may be dealing with the possible depression that comes from a nearly 20 year career of getting constant concussions. Hopefully he does not go the Junior Seau route.