Why does my ex boyfriend still flirt with me?

We broke up 2 years ago, we are still very good friends to this day. I'd say that he is one of my best friends, and he'd say the same for me.

But when we hang out together, he still flirts with me, and stares at me a little longer than normal. It actually gets really awkward for me to keep on looking at him even though we are talking because of how he still looks at me.

So, what exactly is going through his head? I want to ask him, but I know it would end up being an awkward conversation. The only reason why I could think that he stares at me like that is because he's trying to show he's interested in what I'm talking about, but it still weirds me out because the stares tend to last longer than the talking. Still doesn't explain the flirting though.

Oh and I should probably mention that he has a girlfriend.