Jesse had to nap during the day one day. He was going to paint a restaurant during the night while they were closed. Napping was the best he could get. Cooper wouldn't stay still enough for Jesse to nap, so he put him out of the room.
Cooper was so sad about not being able to nap with Jesse, that he just laid by the door with his nose to the space under the door. He was smelling what he thought may help him to understand what this was all about.