“O! Beware, my lord, of jealousy;
It is the green-eyed monster which
doth mock the meat it feeds on."
— William Shakespeare, Othello
I grew up thinking that jealousy was a desirable emotion because it showed that you loved someone and that if one weren't jealous, one didn't care. I've since learned that this line of thinking was purely justification for being jealous.
Here's a beautiful quote about jealousy by Jennifer James:
“Jealousy is simply and clearly the fear that you do not have value. Jealousy scans for evidence to prove the point–that others will be preferred and rewarded more than you. There is only one alternative: Self-value. If you cannot love yourself, you will not believe that you are loved. You will always think it's a mistake or luck. Take your eyes off others and turn the scanner within. Find the seeds of your jealousy, clear the old voices and experiences. Put all the energy into building your personal and emotional security. Then you will be the one others envy, and you can remember the pain and reach out to them."
I have found that jealousy is always about the self and not about the other. Its only value that I see is that it is a step above a more destructive fearful emotion and can be used ever so briefly as a conscious stepping stone toward healthier feelings.