Why does a guy at work flirt with me when he has a girlfriend?

First of all I do like him as a person. He’s kind to me, funny, have tons in common so we get on very well.

I was unaware he even had gotten back with his ex (they broke up 2 years ago and got back together after Christmas)

We have always been kinda flirty with each other, teasing, touching, compliments on my body and looks etc etc.

He still continues to be flirty with me and I tried to back off a bit but I don’t know what to do because I do like him and I do have feelings for him, always have done, and I want to respect that he has a girlfriend but I also don’t want to lose our friendship.
Why does a guy at work flirt with me when he has a girlfriend?
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