The Webster dictionary defines fear as "the emotion experienced in the presence or threat of danger". We fear whatever we perceive as potentially threatening our physical or psychological well-being. With almost no conscious thought, we constantly categorize things, either as safe or as threatening, based on our knowledge of the world and previous experiences. Our judgment is far from infallible, though. More often than not, the anxiety we feel is irrational and rather than caused by real threat is, whether we would like to admit it or not, merely a fear of the unknown. In my opinion, the root cause of fear is ignorance; that is, a lack of knowledge about the entity we feel endangered by.
To begin with, the fear of the unknown is a biological fact. It is no more and no less than an innate reflex that has one purpose: to keep us safe. Thorough our life our brain is trained to discern between what is harmless, and what is dangerous. If we accidentally touched the hot stove burner or iron as a child, that safety center learned from that. If we heard our parents talking about the danger of walking alone after the nightfall with that tone of worry and fear in their voices, that safety center in our head heard it and learned. Alas, most people's safety centers have gone a little too far in their learning. The lesson they probably internalized was "if it is unfamiliar, it may pose a threat. Only trust the things you know."
To follow, the phenomenon of fear induced by ignorance can be easily observed both in individuals and as a social mechanism. The prejudices and phobias, such as xenophobia, homophobia or fear of injections, are more often than not caused by our lack of knowledge and experience. As a fact, people who were raised in a multicultural society do not feel uneasy in the presence of the members of a different race, people who have gay friends are rarely homophobic, and individuals who are forced to take injections