Dog will NOT let his pain be known. He tries to hide his pain and pretend he is normal. Why?

My dog tries to hide his pain and pretends he is normal when he is not. This is very sad for me, because i feel he is trying to fool me. His condition is fragile. He is 14 year old GSD and has arthritis, gastro-intestinal problems, toothaches, irritable eyes (as they get pink on their own). he has weak immunity and is prone to get sick quickly because his natural defenses have deteriorated severely. Despite all this, he still tries to hide his pain. He pretends he is fit and normal, when the truth is painfully obvious. He cannot climb stairs but he forces himself to climb stairs, only to impress me. By doing this, he gives me hurt and pain. He stopped on the second stair, and when i said, "poor boy. i am there for you". He then climbed faster 3 more stairs in excruciating pain and stopped. Then when i lifted him, he tried to resist. It was artificial resistance. His behavior of hiding his pain really hurts me a lot and i want to let him know there is no shame in showing his pain. it just makes me cry