What’s his deal?

There was this guy that I started working with and he was flirt with me a lot he was basically drooling over me he was kind and sweet that’s the only one day was dating another coworker he talks behind her back sometimes. He would deny they were in a relationship (she is kind of a bitch, still wrong though) now he’s flirting a lot of with my friend teases her a lot, Some people suspect he likes her. Now he just acts awkward and serious towards me, even shy I would say. I’m in the process of letting go but I’m wondering what is his deal? He is a hard person to read.
What’s his deal?
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