I wonder if the causation is reversed when people talk about living a shorter life with fewer social connections. If someone has mental health issues or substance abuse problems they're likely burning through their social connections and their health at the same time.

To prove humans need social connection, all you have to do is look at the heartbreaking (and thankfully rare) cases of children that grew up completely neglected and sequestered. As in, locked in a room with no attention for the first decade of their life. When they're discovered, they're not what you think of as human. They never developed language. They never developed the ability to form relationships. They know nothing about the world and have virtually never interacted with anyone. There is no amount of therapy that will ever turn these young people into functional humans and in the case I remember most vividly, they will never develop any real language or ability to communicate either.