I actually think it's kinda the reverse - Eleven's behavior isn't what comes natural to him, it's what he wants to be. He's a lot like Nine, in that he puts such a high premium on manic joy because without that joy he'd fall into a self-destructive depression. So he focuses on the good days, on the quirky surprises that fascinate and amuse him, on anything that evokes a positive emotion in him, really. And he states flat out that companions are integral to that - at his age, there isn't a lot left in the universe that evokes those emotions, but seeing them in the eyes of a friend is just as good. (Another reason his isolation is really bad for him.)
His moments of darkness aren't out-of-character, they're moments when the smiling mask slips.