One of the less fortunate qualities separating us from animals is the way that stress gets stuck in our bodies. Stress is handy for dealing with immediate threats to life or dignity, and if we never experienced it we would have a good deal more in common with cucumbers than with birds or dogs or iguanas. We’d have a little trouble in the motivation department. The issue is that something about consciousness and awareness of the future allows stress to get a bit too cozy in the human nervous system, and drastically overstay its welcome.

If you watch an animal that has been frightened, it is striking how quickly it relaxes again once the threat is gone. To an animal, there is no such thing as anxiety for the future. If the threat is there, the stress turns on; if the threat is not there, the stress turns off, like a valve has closed and the water has stopped flowing. For most humans, the valve never quite closes. It’s a drippy faucet. After a threat is gone, the stress hangs around and haunts us.

It’s like a friend from out of town that comes over for a holiday party and is great fun to have around, and then stretches out on the couch after the rest of the guests have left and before you know it you have an extra housemate.

Nothing wrong with them; it’s just not quite the right time to be hanging around.