Everyone fears death to some degree. Psychologists call the baseline version death anxiety, and it is normal. It surfaces after a diagnosis, after a parent dies, at 3 a.m. for no reason at all. Then it recedes.
Thanatophobia is the clinical name for the fear that does not recede. The word comes from Thanatos, the Greek personification of death. Clinicians class it as a specific phobia, a type of anxiety disorder, when the fear meets defined criteria: it persists for six months or more, it triggers immediate panic when death comes up, it drives avoidance, and it interferes with daily life.
The physical symptoms are the ones panic always produces: heart palpitations, sweating, nausea, shortness of breath, trembling. The psychological symptoms are dread, depression, and obsessive checking of one's own health. People with thanatophobia skip funerals, skip checkups, and change the subject. The avoidance is the diagnostic core. The fear is running the person's decisions.
If you are asking why you are afraid of death, the honest answer is that you are a person who knows they will die. The psychologists behind terror management theory, Sheldon Solomon, Jeff Greenberg, and Tom Pyszczynski, argue that much of human culture exists to keep that awareness at a tolerable distance. The fear is not a malfunction. It is the standard condition of a mind that understands its own ending.
The distinction between the two matters because the responses differ. Ordinary death anxiety responds to the things this article describes: information, conversation, planning. A phobia that is reorganizing your life responds to treatment. If fear of death is keeping you away from hospitals, doctors, or sleep, that is not a character flaw. It is a treatable anxiety disorder.