Why was max acting so depressed at the beach house?

He got his girl back so why? Is it that he was treating his psychokinesis as an affliction instead of a gift? The movie wen't in the completely wrong direction with the powers IMO, he should have had fun with them, I mean he did at first but then he got all depressed for some reason and then it turned into demonic possession? Or does it? I don't know if the movie wants me to believe that toxic waste can make you act demonically possessed or if I was supposed to believe Max was just trolling for some reason. but he handled it in the completely wrong way. I don't know what should've come after it but I think when Mark asked him to pass the potatoes he should have used his power to do just that and thus convince/impress everyone, instead he caused a scene and drove Mark crazy with a desire to avenge his wounded ego. At the end his powers get transferred to Dorita and the movie presents this in such a way as to suggest that this was needed for him to have a "happy ending" I know this post has been sort of a rambling mess, but so was this movie