“Jealousy is all the fun you think they had.” ~E.J

There isn't much to say about Jealousy except that it can stem from a great many things:

Love
Friendship
Beauty
Success
Property

Seeing as how it can apply to so many situations its just a word made up for all the things and times that situations didn't go our way. If you can accept the world for what it is then you can be reasonable

Kants Transcendental Idealism.
"transcendental idealism as a "distinction between the phenomenon and the thing in itself, and a recognition that only the phenomenon is accessible to us" because "we do not know either ourselves or things as they are in themselves, but merely as they appear." (Schopenhauer)

Accept things for what they are in and of themselves not for what you make it up to be.