"Why not?"

Bucky takes the pizza more out of surprise of it being handed to him, than because he actually wants a box of pizza, and he immediately sets it down so that he has his hand free in case Steve does something stupid like run off.

He doesn't get what's happening to Steve at the moment and, quite honestly, it scares him how little like his best friend Steve has been acting since this happened. He's terrified that he's changing on the inside as well as the outside.