We have nothing to fear...

What are some things that you believe are in our nature to fear?
This was the question posed by Julia Ashton. Ever since humankind came out of his caves and developed conscious thought (and probably even before then), they have been fearful of many things. In order to answer what is within our nature to fear, I think we first must answer what is fear? Fear, in my definition springs from conditioning, which is a psychological term which means that one learns one's behavior from ones environment. If one is unfamiliar with these terms, look here for the definition of classical conditioning and operant conditioning. Pretty much, the more one is exposed to a stimulus and an experience of that stimulus, the more one will start associating that stimulus with that experience. Pretty much, if one is exposed to snakes at an early age and one bites them, then they will start to associate snakes with pain which will cause one to avoid seeing them. 
Now, using this theory, what fears I would view to be a part of human nature would be a fear of certain stimuli that have held negative experiences for such a long time that it has become instinctive for us to avoid them. For example, many people fear the dark since one has learned to associate the dark with the unknown (which may contain dangers and predators), Some other fears might be a fear of death, fire, or strangers. All of these things were viewed as dangerous due to their known harmful or unknown natures.
 One thing I have noticed is that humans tend to fear the unknown due to the possible dangers that may lurk there. This even happens today. Why do humans tend to focus on the dangers of the unknown and not on the possible benefits?