I'm not even making a big deal about his expressions and what's behind them, I'm used to that at this point and there's no sense in trying to get to the bottom of it. Maybe it's something, maybe it's nothing. I'm just saying it's not overanalyzing if you notice and comment. They're not micro-expressions or body language. It's conspicuous and people make comments because it's out of the ordinary. When a team scores, usually everyone jumps and laughs and celebrates. But if there's one guy there whose facial expression would have you believe they'd just conceded, of course everyone is going to notice. Don't make someone out to be the bad guy or look like he's overreacting just because he's stating the obvious.
This is nothing new. Had it been a knock-out stage CL match and Neymar just sealed the victory with that goal he would have been happy out of his mind. But it's a league game that they were winning at the time of the goal. He does share the joy of scoring with Neymar, but he isn't flowing over with joy either because he would like to score himself, which he hadn't done at that point in the match yet, and especially after Ronaldo just finished a match where he scored.
Leo cares about that stuff, he wants to follow it up with getting some goals himself, it's completely understandable. He's happy that the team is winning and that Neymar scored, but he was noticably happier after the game when he had scored a couple himself. This sort of competetive attitude is what drives athletes like this, and that's just a good thing. Even in his new role as a provider for the team, he's still a goalscorer at heart, and he really wants to get on the scoresheet in every game, unlike someone like Iniesta or Xavi who is happy "just" to get assists and who celebrate as much as they do whoever scores. This just shows that Leo is not complacent and still hungry.