(SPOILERS)
So! If Sabo has that nasty scar on his face, it's safe to assume he has terrible vision in that eye right? NOW image him smiling at Luffy and he tried to pat his head but misses entirely, trips over, falls on his face and then just sits there like he's a stone figure and suddenly bursts into tears.
The crew are just mortified by the sight of this and Luffy is just pissing himself laughing
(SOMEONE PLEASE DRAW THIS I'M BEGING YOU)