What is exactly going on with your body that makes it so everything hurts less over time?

A few months into training it's blowing my mind. Running the ropes bruised me. All bumps where a shock to my system. Now...nothing. Occasionally something really high or something I'm not expecting jars me a bit, but for the most part just laying in flat backs doesn't really feel like much. Ropes are absolutely nothing.

Physiologically speaking, does anyone know what's going on?