In Plato's The Symposium: humans were originally created with 4 arms, 4 legs, & a head with 2 faces. Fearing their power, Zeus split them into two condemning them to spend their lives in search of their soulmates.my favorite.
I think if Sasuke ever had killed Naruto. Naruto would have definitely kept on smiling. I also think that Naruto smiling after Sasuke killed him would either make Sasuke turns his ways around, or it would fuel his anger even more.