It isn't there.

One day, after I had built up the strength to walk, I went into a nearby suburb to have some time out of the house. Spotting a small shop, I entered and waited to order something to eat.
The girl turned to me and smiled to ask, "What can I do for you, sir?" I returned the smile and started to speak, then the look on her face made me wince. She looked frightened. I asked her for a couple of things, and she got them for me without looking again in my eyes.

When I left the shop, I was hurt and confused. I sat outside to eat. Then I realized what had happened.

A person in constant, chronic pain has a set face that most people read as angry. My smile had been just a thin stretched pair of lips, and it had not touched my eyes. Between that and my size, I had scared this young woman.
This sort of thing happened too often.

I realized that this was why people see old people as unhappy, or mean. They were looking at me the same way. And there really wasn't much I could do about it. The pain was real and deep. It touched my eyes and made me weep. It set my face, at once with pain and determination. It set my gut and twisted my voice around it.
I didn't have the strength to ignore the pain. I'm not sure anyone really does.

What's sad is that people expect those in pain to pretend it isn't there.