The philosopher Kierkegaard would have an answer to this, and I have written a hub about it.
Here is a little excerpt: "Kierkegaard chooses the word "existence" to talk of the special form of life or "way of being" that characterizes humans. In his terminology, only humans exist. Animals and plants do not exist - instead they are just there - but in Kierkegaards sense of the word "exist", the animals do not apply to the definition.
The crucial question is: If existence is not the state of being there, being present, being, then what is it? The answer is, that existence is the same as being, but it is at the same time more than just that - at the same time, we are confronted with the requirement of adapting us to ourselves, so to speak. Human is not automatically itself, it has to become itself by personally overtake or claim itself as the person it really is. Human is supposed to want itself."
Sorry for linking to myself and all, but I thought it was fitting!