It's one thing to argue for improving people's password practices, but please don't pretend that there's no reason for their behavior. The vast majority of people who share passwords between sites experience no repercussions from their choice. And choosing not to create a new password for every site saves them time and potential frustration.
That's the human nature part, to assess the risk of behavior and change it only if future experiences show that the costs associated with that behavior are too high. Since most people don't experience the disadvantages and do experience the benefits this behavior continues.
We can encourage more people to avoid this behavior by explaining the potential impacts and providing accurate estimates of the risk they're taking. We can offer alternatives to password reuse, like using a password manager. But ultimately they are still going to weigh their perception of the risk and benefits to make their own decision.
