It is fair to say that all of us are somewhat afraid of dying and certainly none of us would choose it. However some people take that fear to the extreme and in these cases they are described as suffering from 'thanatophobia' – the full blown phobia of dying and death.
Thanatophobia may result in other conditions such as obsessive compulsive disorder, or hypochondriasis. In the former the person has a series of actions of routines that they feel they 'must' complete in order to feel calm, while in the latter the individual has a constant fear of illness and often carries out self diagnoses with bad outcomes.
To be defined as thanatophobia the fear must begin to interfere with their daily lives by preventing them from engaging in ordinary activities or from living a normal life.
The causes of phobias are not yet fully understood and there is a lot of disagreement between psychologists studying the subject. There is undeniably some 'survival value' in being afraid of certain things (and particularly death) meaning that there is an evolutionary basis for most fears, however this does not explain why only some people hold those phobias and neither does it explain why some phobias are so seemingly irrelevant.
Here are some other explanations.
Freudian psychology describes phobias as being the result of repressed traumatic events. Here the 'super-ego' aims to protect our conscious mind by repressing traumatic events that occurred in our childhood thereby preventing us from having to 'face' them. However this then results in the unconscious mind expressing those fears and anxieties in different ways, and in the case of phobias by 'projecting' those feelings of fear and disgust onto something else that is in some way similar.
A fear of death then might in fact be a fear of something that happened to the patient in childhood – for instance if they fell into a pit they might then associate this pit with a grave – repress the memory and end up projecting their fear onto death and the related paraphernalia. Interestingly Freud also described a Thanatos 'instinct' which he described as the 'death drive' or the 'urge to self-destruction' and the 'return to the inorganic'.
In other words he believed that buried in our psyche each of us harbors some impulse to destroy ourselves (explaining self-destructive behavior, thrill seeking and the impulse to jump from heights). Thus by this logic it would be possible that the psyche could protect you from your own impulse of death and that this is where the phobia could come from.
A more modern approach to psychodynamic theory might focus on our psychology being routed in past (and particularly childhood) events, but ignore the dynamic of the ID, ego and super-ego. This might then explain a phobia of death as being caused by directly witnessing the death of someone close or a particularly graphic death of a stranger (such as a car accident).
Behaviorists describe all of our psychology in terms of input and output. Here it is believed that we form associations between various stimuli, so that we might come to associate the color white with death (as in Japan), or as in Pavlov's famous study dogs could learn to associate the ringing of a bell with food to the point where the bell alone caused salivation.
Thus in behaviorism you would describe a phobia as being an association between a stimulus and a powerful negative emotion or feeling such as fear or pain. We can then 'generalize' this association to other related stimuli – a mental model that is designed to help us survive without having to have too many negative experiences (for the most part this is effective – and you don't need to burn yourself on a lighter and a candle to know that hot things burn).
This could be the result of one very powerful event, or of many events that act as constant reinforcement. For instance then the sight of a corpse might cause us to convulse and fear terror that we then generalize to all things relating to death.
This then also gets us into a negative pattern of behavior wherein we end up avoiding the stimulus and feeling very anxious when we see it. This further enforces our reaction to the stimulus and because we never challenge our fear of death that means that we never break the cycle.
This theory has a lot of credibility, and particularly in the field of phobias and 'desensitization' which is based on this explanation is the main treatment currently used for handling phobias. However further theories and studies have found there to be many other elements in our cognition and this includes fear and phobias.
Cognitive behaviorism is currently the dominating field of psychology and 'cognitive behavioral therapy' has been very effective in treating a range of such phobias. This takes the basic concepts from behaviorism and then adds a 'cognitive' component to it which focuses on our negative thought patterns which we use to frame and contextualize events. For instance in the case of a phobia there is not just the feeling and fear caused by the image, but also the thoughts that surround the event.
For instance in the case of a fear of death we might think things like 'I will be nothingness when I die' or 'Looking at that skull is tempting fate' or 'I could die at any moment' and these thoughts would only serve to worsen the fear. By the same token by rationalizing the fear in a safe environment it is possible to overcome intense feelings of fear and to look at them more logically.
Treatment options for thanatophobia are varied and will depend on which of these descriptions rang most true for you. Seeing a therapist of some kind is certainly a good idea and they will be able to help you come to terms with your feelings. If you see a psychoanalyst then they will talk you through your childhood events and try to uncover uncomfortable and repressed memories that might be manifesting themselves as a fear of death.
Alternatively if you see a behavioral psychologist then you will be able to use desensitization which will involve gradually increasing your exposure to the stimulus that makes you afraid along with constant reinsurance and reward – this is something that works for many people.
CBT or cognitive behavioral therapy will use this same method but will also help you to note the contents of your thoughts and to identify and eliminate negative thought patterns that are making your fear worse and to rationalize your views on death. They may also give you useful tools to help you control your fears such as breathing exercises and positive affirmations.
You may also be offered antidepressants or anti-anxiety medications from your doctor and this can help you to combat anxiety in your every day life. It is important to only use these if you really need them however, and to make sure that you still focus on addressing the root causes of the problem rather than just suppressing the symptoms.
Finally exploring the concept of death with someone close to you and generally meditating on the subject can also be helpful.
If you are religious then often looking at death in the context of those beliefs can be comforting, as can seeing the positive sides of death such as rebirth and the celebration of a loved one's life.