The existentialist generally believes in a sole existence; meaning that we are alone in the world, and that we have no one but ourselves. They also believe in the human being's capacity to feel more than one kind of pain, and that there are three different levels to our pain, physical, psychological, and emotional. All of these types of pain can cause us to become irrational and anxious. But are we really anxious because of this pain, or is it because we ultimately feel alone? And if we are all individuals, can we really feel the same kind of pain as someone else?
Existence precedes essence is also dominant in the mind and thoughts of the existentialist. That is, the idea that we are born into this world without a predetermined nature, and only later in life do we shape out nature through actions and choices. This opposes the traditional view that is that essence precedes existence, according to which we are seen as having a purpose, and values, all of which is determined before birth. (I personally think that the idea of essence preceding existence is rather obscure. There is no way that our values and purpose can be determined by genetic code. DNA is what we are, not who we are.)
Existentialists pose another question to the opposition: how can we be happy in a world devoid of significance and meaning? The loss of external values allows us to derive value from within ourselves. This value is greater than any imposed by outside forces, and thus cannot be