Sad. It's the upper eyelids, giving the impression of sagging eye corners. That reads as "sad". Furthermore, you gave him two highlights in the eyes, both of them looking rather oval than round. That, too, translates as "sad" in human eyes (in fact, it translates to wet eyes like in "swimming in tears").
But he's cute nevertheless, but if you want him to get rid of that sad look, you'll have to change these.
Just a quick example: get rid of one highlight per eye, make highlights rounded, get rid of "eyelids" and accentuation: