They have low self-esteem. What one person looks like or how much money they have should have no bearing on you.
To a certain extent it's human nature for people to want to (compare) themselves. There is always a pecking order even if it imaginary. The ego is a fascinating device. People would storm the streets in the U.S. if all schools and colleges switched to a "pass and fail" grading system for example. In other words there would be no difference between someone who got a D and someone who got an A. They'd both have a pass average. (Equality equals Mediocrity).
Everyone wants to be BETTER than average.
We want to know who is #1 for any category, we want to travel "first class", dine in 5 star restaurants and stay in 5 star hotels.
In the U.S. those with money or good looks are "privileged" in our society. Average people become frustrated watching such people get breaks in life simply because they were born into a certain family, had great genes, or just got a "lucky".
However most "haters" despise it when they (believe) they are just as smart, talented, or good looking as those they dislike BUT they never got the breaks/opportunities or (received validation) from the outside world. What they're really mad it is GOD or LIFE for not being "fair"!