Our (and Luke's) original impression of Yoda is by no means flattering. He looks like a crazy hungry green thing that unfortunately has the ability to speak. Yoda digs through Luke's stuff, eats Luke's food, and, obsessed with that awesome little flashlight, ends up stealing it. (He even goes so far as to fight with Artoo over it.)
So was Yoda slowly losing it during all those years as a hermit?
Perhaps going from fighting battles and conversing with others on a daily basis to living in total isolation with no one to speak with would play a part in that? Or could Yoda have been faking his apparent insanity for some reason? (If so, what is mentioned reason?)