What does it mean when a guy smiles at you but the smile is crooked?

[Casper286]

I was talking to this guy that I think I might like and every time I told a joke (I'm usually very funny) he'd either laugh or smile but he smiled with only half his mouth like this: https://i.imgur.com/DXn70dc.jpg
What does it mean? Was he flirting with me or did he not find my joke funny? Or was it both?

[Anonymous]

In my opinion it's a fake smile. He doesn't really think the joke is funny but tries to play it like it is so he doesn't hurt your feelings.

[Anonymous]

It's probably a gas pain.