It makes perfect sense that if everyone is jockeying for position at the top of the heap, the achievements of neighbors, friends, and even family members will occasion jealousy. No holds are barred in Vanity Fair and no relationships are too sacred to be spared brutally honest treatment. Sons are sexually jealous of fathers, sisters and brothers are financially jealous of each other, and people form deep friendships only to immediately dissolve them when their relative ranks shift slightly.
In the novel's world, jealousy is the most normal emotion one human being can have toward another, since without it the intricate social network would collapse. Even other feelings are usually clouded by or somehow bumping against jealousy. Those who lack the capacity for jealousy are not full participants in life.
Often characters are placed in situations where they must choose to either display or repress their feelings of jealousy and other emotions. The ability to see these situations as choices, and to make the strategically appropriate one, is a necessary quality for getting ahead in the social hierarchy.