He's missing the passion you two had in the past and to get that feeling of being wanted again, rather than trying to work on it he is trying to bring it out of you through making you jealous. I had an ex that did this. It's completely immature, but it makes sense and once you understand you can do something about it.
I know you love him and in your mind you're thinking, 'well of course I love him. He knows that". He's not feeling it. Not in the same way when you two got together. Now I'm assuming he's not just being an asshole and flirting with other girls because of that fact. But from how you described it, I think he's trying to get that rush of emotions in you that he misses. Getting jealous is actually showing him you care about him.
If your devotion to the situation is known to him, and if he still feels the need to play games, Then what you are dealing with is an egotistical douche nozzle. I would recommend maybe redefining your relationship with him. If he knew of your feelings for him, Then the games should have stopped by now. I don't think he should have played them to begin with as I am a firm believer in the straight forward and honest approach without all the bullshit.
Why do you fall for it? Ignore it or better it. I don't know what he's doing to try make you jealous if its flirting with girls I'd just walk off and start chatting to a guy or leave text him you've gone to a pub with a friend you met there and you'll see him later. Take back some damn control
You shouldn't care about why is doing what. As long as you 2 are not in a relationship it doesn't matter what he wants or does. Cut him off your life and focus on finding somebody who wants you rather than playing games with you.