Why would a guy flirt with me when he knows I’m married?

A guy I know has started flirting with me quite a lot, why does he bother when he knows I’m married. He does the following:

Checks me in regularly

stares and engages in locked eye contact, he doesn’t look away. What does that mean?
Light teasing/banter

Sits really close to me when talking, leans right in.

At times gets a bit nervous and fidgets when one to one

stutters at times when talking one to one, like he looks at me (stops talking) recollects his thoughts and talks again.

Why would a guy flirt with me when he knows I’m married?
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