If I did this and the relationship ends 20 years from now I could find myself being an old guy without any friends and then it's harder (but still possible) to find new friends. (because you get rusty when you didn't do this kind of thing for decades)

I can definitely see why some people then choose to sit in front of a TV all day, because with your friends probably also a lot of the activities and hobbies you once enjoyed are gone.