I never considered depression as jealousy. But it could trigger it. Though, to some people it might n't exactly be caused by jealousy rather self esteem. Feeling insecure. But I like the comparison and also I will be thinking about this for sometime.
What I like about this is that you have made your title the question and then answered it within the poem and in a few short lines you have summed it up. The horrible feeling you get when you just want to be happy and it seems like every one around you is happy but it someting you will never have.