Thor attempts a smile -- he truly does -- because he knows that if he doesn't try to smile he's going to end up crying and maybe that's the appropriate emotional response when your brother repeats some of his last words to you, but it's not the emotional state Thor wants to be in right now.

"I want to believe that," he says. "But I'm not sure that I can." If he goes home with that hope and nothing comes of it, it will be worse than if he had no hope at all.