Shiva is the source of Yoga, Meditation, Celibacy and Penance. He is free from all the materialistic desires and fashions. He is complete and absolutely perfect.
Shiva teaches us to live a simple life without expectations and desires. His message includes a fearless life and a fearless death. No love and affection for the mortal body.
The ash is the reality. It is the only thing which remains within the 5 elements of nature. Since this body is made up of sky, water, air, earth and fire, in the end the only thing remains is ‘ash’.
To signify the truth of life, Shiva put ash on his body. Also on Kailash there use to be an extreme cold. So to keep the body warm, sages use ash and cover their bodies.