It might also be something like: people who experience relatively little psychological distress are optimistic, and also live longer.

We don't like to talk about it, but there is a biological component to how happy or anxious someone is. Not to say that everyone is doomed to be a certain way because of their genetics, but I've known people who are basically never sad, and it's not because they've found some kind of secret of happiness. I have a friend who has the temperament of a golden retriever.