He knows the universe secretly loves a good misunderstanding. It must - the artful way they're constructed, so perfectly, suggests a malign humor.
He's smiling, in part, to pretend he's in on the cosmic inside joke. "HA! Good one, Universe!"
Mainly he's smiling because it's raining.
And were he not smiling the rain, combined with an unexpected personal ache and sadness, would make him even more ridiculous than he is already.
And also...that nose. I mean look at the size of that thing.