EXISTENTIAL ANXIETY (or existential angst) is a term used to describe the anxiety and fear one feels when burdened with the responsibility of freedom. In his book 'The Concept of Anxiety' philosopher Soren Kierkegaard describes it as a "Dizziness of Freedom".
For example, if you were to stand on top of a cliff it may suddenly dawn upon you that nothing is stopping you from jumping off that cliff - you are free to make your own choice - you may choose to jump or you may choose not to jump. Either way, the responsibility is yours, and you are the only one to blame for the consequences.
So while freedom is a quality that many of us wish to have, for some of us the idea of being responsible for your own actions may be a daunting prospect as we live in such a cruel and indifferent society - because of this we try to shelter ourselves away from the truth of responsibility and all the hazards that the outside world has to offer.
"Man kind does not feel at home in this world, there is no safety in the world" - Paul Tillich, philosopher.
(Source(s)http://www.thefreedictionary.com/existentialism http://www2.webster.edu/~corbetre/philosophy/existentialism/whatis.html )