Rob just didn't like being cold. Though movement usually kept a person warm enough and dueling definitely required a lot of movement.
"I am, too," Rob said as she bumped into him. He grinned and moved a bit to accommodate her while the other person walked by. They were coming upon the split for either the Ravenclaw tower or the Gryffindor tower. "Are you sure you won't starve before breakfast?" he asked one final time, "I don't think it would do me much good to have a Dueling opponent who faints due to hunger." He was smiling while he said this to show that he was just teasing.