Hugging someone is a good way to show appreciation.
Brian had paid for Jeff's lunch when they went out to eat.
Jeff wants to show his appreciation.
Jeff gives Brian a hug and tells him thank you before they leave the restaurant.
Jeff slaps Brian on the butt before they leave the restaurant.
Brian is happy to take Jeff out to eat again in the future.
Brian is freaked out by Jeff's action and doesn't invite him out to eat again.