I feel like I can be very jealous too, and I hate it! Sometimes I can rationalise it away - for example, this afternoon I saw one of my favourite chefs (Heston Blumenthal) on tv, giving a dinner party and serving his signature creative dishes. I felt jealous of the guests, but realised there was no way I’d have got the chance to go to said dinner, even if I’d known about it. I wasn’t jealous they were with Heston, just that they were getting to try his creations.
But, it’s a lot more difficult to rationalise the feeling away in a relationship. I’ve read it comes from a place of insecurity and fear. It certainly does, in my case.