I've experienced this from both sides, yet I can't quite understand the psychology around it. A member of a family who has been single (typically a younger sibling) begins dating and bringing over a new partner to the house to meet everyone, and they are all kind of lukewarm on this new person, maybe a bit cold at first. Any negative traits of this new person will be pointed out behind their backs. Eventually, the family may warm up to them but that takes much time and effort. And any little mess up in the relationship will instantly shoot them back to square one again. This phenomenon seems extremely common to me. But why is it?