Why is he trying to make me jealous?

I saw your old question in the Horoscope section.

I guess many are like that because they're insecure...

and it's a way to see if the person will notice and seem to care.

They might like someone and act flirty with someone else to try to get some sort of reaction... That kind of behaviour is probably an attempt to get signs that you like him back or cares about if he's flirting (being affectionate) with someone. To Pisceans it can be a good sign if the person they like notice that they're flirting with someone else and doesn't seem to like it, because Pisceans feel a bit liked by it. Also some could like provoking some jealousy because Pisces is a rather passive, emotional and soft sun sign. And jealousy is a strong and kind of passionate emotion. Pisceans want to be liked/loved.

They're also insecure (as I mentioned), they're not sure if the person truly likes them back. One day they might feel rather confident, but another day they could feel very insecure and have sucky confidence.

But I also think some Pisceans aren't sure how to act, at least from time to time. They can be shy and insecure around the person they like, at times they just back off... As it's usually not a sun sign that goes after what they want, provoking a reacton could give a slight confidence boost, as it's almost as if provoking some reaction makes them feel a bit liked. It can also make them stronger and more daring as they usually want signs that the person is interested before pursuing her/him. Some need a lot of signs before they dare to do anything, some might even wait for the person they like to go after them first.

He likes you.