Why did he do this mid conversation?

I like this guy at work. I can’t confirm that he likes me back but from my standpoint, we have chemistry and we flirt a lot (more so without making it obvious… we make intense eye contact, stand really close to one another, fingers touch when he hands things to me etc…) but it’s an unspoken kind of situationship where if he truly had feelings for me and we dated, our jobs could be affected so we just keep it subtle. Anyways, the other day we were talking. Like usual we were making direct eye contact. The thing that threw me off guard was that during that conversation, there came a point where he slowed down what he was saying, stopped, smiled (more so like his face was in awe not like a cheesy smile), and then went back to what he was saying. He’s never done that. It wasn’t a funny conversation or anything like that… what do you think was going on in his head?
Why did he do this mid conversation?
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