I've always wanted to say this...go on ahead and leave this one to me, got it?
I like how the anime casually forgot to mention the fact that he’s drunk during this scene and that’s the reason why he slaps himself
Yup. It says:
"Shizuo looked like he’d had quite a bit to drink, probably at Simon’s sushi place."
And then during this scene:
His palm stopped right in front of her helmet, and he was smiling as if nothing had happened:
“Ah, I’m OK. Luckily I haven’t sobered up, so it doesn’t hurt that much. You go on ahead, I’m alright. I don’t get really what’s going on, but you need to go after that girl right?”
Then, Shizuo folded up his shades, slipped them into his breast pocket and slapped his cheek with one hand.
“Heh heh, I’ve always wanted to say this: ‘You go on ahead, leave this to me!’”
Yeah he’s drunk.