What are 2
factors that prevents joints from rubbing and wearing down your bones

A layer of smooth cartilage at their ends, which cushions the bones and prevents them from rubbing directly against one another. The whole joint is also enclosed in a synovial capsule, which is full of synovial fluid. The synovial fluid acts like the oil in an engine, lubricating the joint and preventing the cartilage from becoming worn down.

The way joints are made, they naturally should not have trouble with west and tear. The other factor is the cartilage which acts as a kind of shield for the bone and this keeps you from getting arthritis unless you are unhealthy