It is generally accepted that phobias arise from a combination of external events and internal predispositions. Some phobias such as arachnaphobia (fear of spiders) and ophidiophobia (fear of snakes) however, may arise more easily due to an evolutionary trait that conditioned humans to fear certain creatures that could cause them harm.
In a famous experiment, Martin Seligman used classical conditioning to establish phobias of snakes and flowers. The results of the experiment showed that it took far fewer shocks to create an adverse response to a picture of a snake than to a picture of a flower, leading to the conclusion that certain objects may have a genetic predisposition to being associated with fear.
Many specific phobias can be traced back to a specific triggering event, usually a traumatic experience at an early age. Social phobias and agoraphobia have more complex causes that are not entirely known at this time. It is believed that heredity, genetics, and brain chemistry combine with life-experiences to play a major role in the development of anxiety disorders and phobias.
Having a phobia stops you from having a full and happy life, I should know, I spent 13 years not driving or going to the dentist because of the template I had well and truely fixed in my brain.
The parts of the brain responsible for this is the Amygdala and the Hippocampus. The Amygdala looks out for your safety, it's responsible for your fight or flight response. Take the Amygdala away and you walk under buses or do something really dangerous because you've got no health and safety executive telling you to stop.
The next stage of a phobia is to lay the response down in a template in the Hippocampus, the Amygdala responds to a stimulant and the hippocampus tells it how it dealt with it the last time - in the case of spiders - usually screaming. In my driving phobia it was just freezing and not being able to move. This was built up over years of having bad experiences until my brain just refused to engage with the situation - result - panic attacks and freezing every time I sat in the driving seat.
The good news is that Phobias can be managed and once under control you can manage them until they have gone, and they do go, especially if you have a real vision of how you'd like your life to be if the problem wasn't there. I basically decided that I needed my independence back.
Starting with relaxation and using a fast phobia technique, and reframing the problem, phobias usually take 4 or 5 sessions to get under control. If there are more or anxiety is generally very high, it might take longer. I have seen people with all kinds of phobias, from toadstools to lifts, from driving on the motorway to dentists, with good success rates.
Also see: A-Z of phobias