What does it mean when a guy smiles to himself after looking at you?

There have been plenty of interactions between me and my crush, including him saying good morning to me every day at work. And most of the time, I will catch him gazing at me.

However, I’ve seen him look at me (when he thinks I weren’t looking) and then smile to himself. This has happened three times recently

1 - I had sunglasses on so he didn’t know I was looking at him. I walked past him and he turned around to look, then smiled to himself briefly

2 - I had my sunglasses on again and was looking at him. He was walking towards me while I was sat with colleagues. He gazed at me as he came towards me, then his eyes went to the floor and he smiled with a closed mouth, the corners of his lips stretched downwards. He then kept his eyes to the ground as he walked past me

3 - I was sat on a bench, with my back to him. I was sat a fair distance away from him. I heard him pushing a trolley and I turned around to see who it was. It was him, and when I turned around, he was already staring and smiling at me. I looked away and then looked back at him, and he was still staring and smiling and he waved at me

What does it mean when a guy smiles to himself after looking at you?
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