My son and I were at the LGS and I asked to look at something. The salesman took the gun from the rack and cleared it before handing it to me. I cleared it as soon as it was handed to me. My son asked why I did that and here's what I told him.
"When I saw the man across the counter clear the gun, it gave me confidence that he knew how to handle it safely. When he saw me do it, it gave him confidence that I could also be trusted. It's kind-of like a secret handshake."
When I was a teenager, my cousin and I were camping and we had my old Marlin model 60. It was leaning against a tree and I went to pick it up. He told me that he had unloaded it. I was much more familiar with that gun since it was mine and I knew that if he had simply dumped the rounds out of the magazine that there would still be one in the chamber and one or two more left in the action. I aimed at a can at the bottom of a dirt bank and fired twice before the gun was really empty.
To a much greater extent than most mechanical devices, firearms are terribly unforgiving of any overconfidence, complacency or negligence.