The physical in the existence is constantly under threat because the physical is always within limited boundaries – it is always demarcated. With anything which is demarcated, there is always a fear of losing it. For instance, this physical “you” is constantly under threat, no matter how healthy, strong or young you are right now.
Fear has become so fundamental simply because your experience of life has not gone beyond the physical. If we had explored and established ourselves in other dimensions of experience, the body would not be such a big issue. But now, no matter what kind of teachings other people give you, whether someone tells you that you are atman, paramatman or whatever else, your whole experience is limited to the physical body. So the fear of losing it is natural. But if you establish yourself in other dimensions of experience, the body becomes an easy thing to handle. Life or death won’t make such a big difference.