Oldest children are oftentimes the most jealous, even envious. Oldest children are the ones who are dethroned, displaced, & discarded in favor of younger siblings who parents feel need them more. Oldest children are the ones who parents treat the most differentially in the harshest way. Oldest children are held to strictest & toughest standards on the basis of their ordinal birth order. It is always assumed that oldest children should know better & oftentimes are given no quarter if they fail or make mistakes. Oldest children are given stiffer penalties in comparison to younger siblings at the same age for the same infractions. Also, oldest children see how their younger siblings are treated in comparison to them. They see their younger siblings having an easier life as children & teenagers than they had as children & teenagers.
Only children are the least jealous & envious of all birth orders. They are never dethroned, displaced & discarded in comparison to the oldest child. They are at the center of their parents' universe. They also have the most individualized parental time & attention. They have the best of all worlds. Besides having parental attention & resources to develop themselves, they don't have to compete w/siblings for parental attention, time, & resources. They also have an unencumbered childhood as opposed to oldest children who oftentimes have the shortest childhood as they must be adults quickly.
Only children have full opportunity to explore who they want to be. They have myriad opportunities to be the best people they can be. They aren't pushed to be adults but are adult-like because their primary interactors are parents who have more experience, education, & knowledge than mere siblings. Because they don't have to endlessly compete w/siblings, they are secure & confident in their being. They are also more adjusted because they don't have to endure siblings & the gamesmanship that is associated w/having siblings. Again, only children have THE BEST of all worlds & are the happiest of all birth orders.