Why did they pick 6 years as the measurement for a successful relationship? Given a perfectly justified skepticism of social science studies, that number seems

Why did they pick 6 years as the measurement for a successful relationship? Given a perfectly justified skepticism of social science studies, that number seems very much like sample retrofitting. Beyond that, even their hypothesis is absurd as it implies that the way a person interacts is a causal factor for a relationship, yet the way people interact with their partner is quite obviously caused by how they get along! It's like saying people who enjoy going to the beach are often tanned, and implying that if you're not tanned that is a causal relationship with you not going to the beach. It's really bizarre logic.