He'll look at the ground and smile every now and then. Why does he do that?

Kamio
Whenever I am talking to this guy (we recently become close), he'll look at the ground and smile every now and then. It is this little discreet smile and it is driving me crazy!

He'll smile like that when I crack a corny joke or ask him about himself. It is like he has to pause, smile at himself and then answer my question.

There are other things that make me wonder if he is into me...but right now, I wanted to know 'What on earth is going on behind that smile?'
He'll look at the ground and smile every now and then. Why does he do that?
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