Why did Doug smile when Lucas ripped his art?

Doug smiles because Lucas didn't notice that Doug was wearing Joe's jacket.
Doug is given a gift of a New York Yankee's jacket that belonged to Joe Pepitone. He's immediately worried that his brother will see it. He thinks to himself that Lucas will steal it if he realizes who the jacket belongs to. Doug also thinks to himself that Lucas will definitely find out because his brother always manages to ferret out secrets.
One day, Lucas comes in and sees Doug drawing. He starts to comment on the jacket before his attention is pulled away by the puffins that Doug is drawing. Lucas mocks him and rips up his art, but he doesn't notice the jacket. Because of this, Doug is smiling when he goes to hide the jacket under the stairs again.