If I was not jealous then I would question how much they mean to me. I would question how much I mean to them if they were not jealous sometimes too.
I got into a fight with my girlfriend a few weeks ago because after a dream she brought up the poly thing, and I said I wouldn't be jealous if she slept with someone else but would be upset that she did so when I'm not allowed. She got upset that I wouldn't be jealous, and I really didn't understand... and still don't.
I think on a fundamental level I don't understand jealousy the way most people do. To me it's just a negative emotion...
I would never think I didn't care about someone just because I wasn't jealous. I think I'd only be jealous if I couldn't "partake," as it were. I judge how much I care about someone based on what I feel like doing for them. I find myself doing more for her than I would for myself, and giving up time with friends or other things I enjoy. What should her spending an hour with someone else matter?