How do people get phobias?
An irrational fear of something is a learned behaviour, sometimes from a parent or a respected friend or relative. It is part of the body’s natural defence system: often phobias are exaggerated fears of evolutionary memories; imprints on our brains that help protect us through caution, making us innately awareness of something that could harm us.
For example, we know there are poisonous spiders in the world that can hurt us and even kill us, so we have an awareness of that on a primal level. However, a house spider in the UK is completely harmless. A person with a phobia is unable to make this differentiation.
As a small child we learn about fears from our parents, as they teach us (consciously and unconsciously) how to live and relate to the world around us. If you see your parent being frightened of a spider, your instinctive response and the message hardwired to your brain is that this is a threat and can harm you too.
Over time, when you see a spider that message is reinforced, so it is exposure to that fear which compounds it and makes it worse. (Taken from an article published by NCH)