Antisthenes used to say that envious people were devoured by their own disposition, just as iron is by rust. Envy of others comes from comparing what they have with what the envious person has, rather than the envious person realising they have more than what they could have and certainly more than some others and being grateful. It is really just an inability to get a correct perspective on their lives.
DiogenesI am called a dog because I fawn on those who give me anything, I yelp at those who refuse, and I set my teeth in rascals.
Diogenes