Given their low self-esteem, it aggrieves them to see someone else doing well, in any sense of the word. Their jealousy causes them to lash out. Their aim is to bring the other person down to their level by belittling their successes or happiness. Yes, it’s spiteful, but it’s the only way they know how to approach those people who have what they want. The underlying message is: “If I can’t feel good about myself, neither can you.”