I smile when I'm outside at night in the wilderness, for 2 reasons. 1 good reason and 1 shitty reason. I'll tell you the shitty reason first: 0) So strangers who I might happen to see will think I'm a nice person, which decreases my chances of getting accidentally shot. 1) So my teeth are seen from a distance by a particular kind of large feline (being feline gives them good night vision, so even though I can't see them they can see me). The teeth serve as a way of saying "I will [attempt to] defend myself" (I am an animal that WILL put up a fight). Hopefully this encourages them to pick an easier target.