Pain is such a relative thing.
There's nothing more challenging than working with people for whom the fear of pain is so great that they avoid anything that has the possiblity of hurting them.
On the other hand, there's nothing more rewarding than seeing people who are dealing with serious physical obstacles, and who push themselves right to the line (sometimes beyond it) because the other choice - avoidance - is not an option.
Two examples of the latter come to mind.
I currently work with someone who has had spondylolisthesis (you can look that one up) and arthritis in his shoulders. I've researched the conditions, spoken to physiotherapists, and laid out a general plan to get him moving forward. Everytime we push a little further, I ask him how he's doing - and pretty much get the same answer. "I'm hard to kill - don't worry, I'll let you know". The other guy I used to work with suffered from Thoracic Outlet Syndrome - again, I'll let you look it up. But suffice it to say a) it can be debilitating, and b) he's got one of the worst cases I've seen. And every day, he'd come in the gym and I'd cautiously try to help him move forward... and almost inevitably, as we were stretching at the end he'd tell me about work that he did around the house the previous weekend that was infinitely higher risk than what we were doing in the facility. When I'd point this out, he'd shrug - things have to get done. Life has to get done - or we stop living.
I hope that if I'm ever faced with adversity like these two, that I'm able to keep the same positive attitude - and, more importantly, that I have enough courage to recognize that sometimes living in fear of pain is worse than the pain itself.