I've always envied people who had not just siblings (because, yeah, learning to have another person in your space is hard), but siblings of the opposite sex. Growing up with an understanding of how the opposite sex thinks/feels about their body, about friendships, how they deal with fears and insecurities, and react to power-struggles among their peers... all that shit is a complete fucking mystery to only-children, and it's an complete mind-fuck when you run head on into it in your first relationship.