What Kuma did was temporary. Luffy was sad and confused, and it was, of course, very hard to watch this man who loves his crew like a family watch them all disappear, unsure if he was ever going to see him again. It was not the most sorrowful moment here, though. Ace will never come back, and unlike the situation with Kuma, Luffy new this much. He was in such immense shock from Ace's death that he could hardly bring himself to move and had to be dragged off of the battlefield. Not sure why this is a close vote.