Sometimes Nick feels invisible. It's not a comfortable feeling, which is why he overreacts, makes jokes that fall flat, flirts with Sara, competes with Warrick.
It's not intentional. He knows that the frat-boy routine bugs the Bugman but it's a comfortable role to slip into when he's feeling unsure. He thinks Catherine might get it, although he can't imagine Catherine ever thinking she doesn't matter.
She gives him a wry look, tells him he needs to figure out who he is. He knows who he is.
He's Nick Stokes, CSI.
It's as good a mask as any other.