Fear is usually associated with a pair of almond- shaped structures in the brain called the amygdalae. The amygdalae have a lot of connections with the part of your brain associated with memory. In general we're very good at building connections between objects or situations and fear following a negative experience. For example, if someone shakes your hand with an electric shock toy hidden in their palm, you're like to remember not to shake their hand again-- even years later! You'll remember this better than you would a memory that isn't associated with fear. We also seem to have a lot of instinctive fears that probably helped us survive in our evolutionary history, such as a fear of snakes or disease.