He has always been this mopey, never seen a player with a weaker attitude than him. Listening to his longer interviews makes it all clear. He has very little fight in him and blames everything on other people.
I think he's mentioned psychological issues in the past. Wouldn't surprise me if he comes out one day and talks about suffering with his mental health, he's very streaky in his form but also always was a better reactive striker than a clinical one.
Unfortunately I think any player with these characteristics is going to struggle to play at the really top level in any professional sport. As it is his speed and directness has carved out a career for him. By the time his legs have gone he could end up like Fernando Torres at the end of his career.