Guy's Behavior

Why is a coworker suddenly saying hi/bye and making small talk?

I came back from a break from work after some weeks/months and the first day he saw me after that he acted shocked to see me and said some along the lines of “I know we never really talked much but feels like I haven't seen you in a while” and asked me about why I was gone etc. mind you this guy had trouble even saying hello to me in the past when I would try to start saying hi every morning, he wouldn’t keep it up so in the past I laid my feelings for him to rest thinking he clearly wasn’t feeling me back.
ever since I came back he says hi and even said bye and made small talk about the weather. All of this is shocking to me. I had been a coworker to him (different department) for 2 years prior. Forgive me for being confused by the sudden friendliness. He’s low key but social and dare I say charismatic in his own quiet way. He’s also selflessly helpful to everyone all the time so that helps make him attractive too.

Why is a coworker suddenly saying hi/bye and making small talk?
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