Being the privileged ones who have been bestowed with the power to reason, even then, Humans lack intensity in whatever they do. If this wouldn't have been the case, there would be no need to work towards the ultimate for a lifetime. However, every person touches the level of intensity at the time of death, which is inevitable to all. In no other thing do they ever experience this intensity during their lifetime.
In order to explain this, Lord Shiva went and sat in the cremation ground or Kayanta, waiting for the body to end. Kayanta, where ‘Kaya’ means body and ‘anta’ means ending, where the body ends, not where life ends. Hence, it is the end of the body, kayanta, and not the end of life, jeevanta. We all know that we cannot carry anything with us on our last journey. Hence, if it is the body which you have always given significance to, then the time of kayanta becomes the most intense moment of life for you. Parting with the physical self and all materialistic things which you have earned in your lifetime becomes the most intense moment as it is of great significance to you. However, on the contrary, if you have always thought and known something beyond the body, and have realised the nature of who you are and where do you belong, then death or kayanta is just another moment for you. It is not of great significance at all.
We are all mortals. The end of physical body is sure to come to everyone, but if you are not just a living body but a living being then immotality is just a natural state for you.
As for Shiva, when he sits on the cremation ground, watching all the drama going on with the immortal humans, he achieves enlightenment which gives way to realisation and is not an accomplishment or achievement. It is all about your perception, being equipped with your pragna and not with your senses alone. This would enable you to connect not just with your kaya but also with your Jeeva, and you would become immortal naturally.
Sham means shava or corpse and Shan refers to Ananya or bed. Shiva began residing at the kayanta or Shamshan, where the dead bodies lie, as he realised that working with the living people is a complete waste of time. They can never achieve that pitch of intensity which is required to understand the realms of life and beyond it.
Intensity is not an instinct of survival. In fact, these are two different fundamental forces. One being the instinct of survival and the other the longing to expand limitlessly. Mastering the former only plays low on you as survival itself means playing safe. But if you seek towards limitless expansion and focus all your energy towards it, you would achieve what is called full intensity in your life.
Every creature has the instinct for survival predominant in them, however as humans evolved more and more over time they were granted with a higher level of intelligence and awareness which enables them to keep the instinct for survival at bay and work towards expansion. Out of these two forces, one is always trying to fuel and gear up the intensity in you while the other in always pulling you down.
When I connect Shiva with Shamshan, I am just trying to portray that the lord is sitting at a place where life makes utmost sense because there is much more to life after death. But if you are on survival and self-preservatory mode, your body would be your whole and sole concern and hence death would seem the end for you but if you pursue the limitless expansion for yourself even after death, it would be a new beginning for you and the lord is simply waiting at the Shamshan to welcome you to a new chapter of life, life beyond death.