What are you smirking at? Did something good happen?

Tom's grinning.

What are you grinning for like that? It creeps me out.

Who do you think you are to be grinning like that?

"Why are you grinning like that?" "That female high-school student I just passed by was really cute."

That person has had an odd grin on his face for a while. What do you suppose is behind it?