Why is a married man whom I've known for years flirting with me?

So a married man, quite a bit older than me (24 years), seems to be giving off flirting signals as of lately:

He touches me on my back, shoulders, hands, sometimes it is obvious, sometimes he makes it look like it happened accidently. Then he compliments and teases me and tries to make me laugh. He has started to initiate hugs when we see each other, but not in front of other people. He's indirectly asked if I have a boyfriend. He has called me cute and charming and smirks and laughs at things I do or say that he finds cute. He basically always provokes some kind of cutesy reaction out of me.
He told me I mean something to him and is always offering his help. He comes extremely close to me when we talk or at any given situation, especially when no one else is there.
He saw me with a guy recently and involved me in a conversation, totally leaving the poor guy out, so that he went outside on his own and waited there for me until the conversation ended. He also treated another guy who showed interest in me like an ass.
He always brags a lot and checks me out when he thinks I don't notice it. Once I looked at him accidently, he was startled, then looked away and smirked. He is protective of me, has told me personal things about him and asks questions about my life, feeling empathy for my stories. He is self-conscious around me and notices even the smallest things about me. And he has given me kind and patient advice on things. And he laughs a lot.

I don't think I'm imagining things or overinterpreting. I think he is flirting or what you may call it. My question is, is it just for fun or does he see me as "affair-material"?
I never thought that far because I've known him since I was a kid, so I thought he'd always continue to see me like that. But now I'm not so sure anymore.

I'm not being too full of myself, am I?
What is happening?
Why is a married man whom I've known for years flirting with me?
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