Most Helpful Guy
"Did he not want to see me" probably opposite of that. He was taken a back when seeing you making him stop smiling. He was probably smiling because of something funny and you distracted him from it and that removed his smile.
I wouldn't be too worried. You obviously like him. Try get in contact with him, I'm pretty sure he would not mind.
Don't analyze everything this deeply, you will miss the obvious things when you focus on the small things. You just caught him off guard and I would say it was a positive reaction. If he wasn't smiling to begin with and then didn't smile seeing you, that would be different. I could bet that if you see him and he doesn't smile he will when he sees you.