I was having this conversation just recently. People who don't suffer some kind of chronic illness or pain have no idea. Even those who live with us. My wife see's me suffer, she knows how bad I tell her I hurt and she can see the evidence of it, but even at that, while she may be a witness, she doesn't experience it herself.
No it wasn't her I was discussing it with, it was someone else. I had an outright argument with someone once about that very thing. She has a disabled child and yes she witnesses it and suffers along with her, but she doesn't experience it herself and she might think she knows what the daughter goes through, but unless she suffers that same pain, she really doesn't.
That run I'm building? I had half that done two weeks ago. Took me all day today to do the other half, and this evening I can barely walk. But those days I can get out and do stuff, I do it. I'm gonna hurt anyway, I might as well try to accomplish something, so what if I don't get dressed and leave the house for a few days after