Why does he care? Well, that's a good question. It's just odd! And like a trainwreck, he can't help but stare and be more interested. However, Crow responds to Gokudera in the same way, his tone getting louder and he becomes annoyed himself.

"Tch. Like I care if you have you have a target on your back! Better hope that skull doesn't reveal anything else embarrassing!"

It's not like Crow pointing and laughing was helping silver haired brat in that regard anyway.