I think is because of the feeling of being inadequate.
You feel that you are not as good as someone. Thats why you are envious of that someone being better than u in all ways you could imagine.
But... At the end of the day always remember that you must be proud of who you are (after so much you have gone through).
You never know. Perhaps when you are envying someone, another person may be envying you.
;)
