Jealousy happens when we are insecure. It comes from the insecurity about not being good enough, attractive enough, or intelligent enough. And we all know that jealousy is the thief of joy.
Though limerence by its very nature is relatively short-lived (typically lasting somewhere between 3 months and 36 months) it is a very real and intense emotion/feeling. When a person decides to leave a limerent relationship, or if they are the one abandoned by the other, the emotions don't immediately disappear.