h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The obvious answers to "Why are we here?" include

Where else should we be?
Why not?
Ask youre parents

'In the beginning there was nothing but God. He looked around and seeing there was nothing else He shouted "It is I." Then, being alone, He was afraid. Then realizing there was nothing else in existance to fear, the fear stopped. Having realized He was alone, He realized that He could not fulfill His desire. So He divided into two...'

Having thus divided into the abstract and physical universes, the union which once was God now was separate and apart. Fear now was justified but desire was still unfulfilled. So, the abstract god of the heavens, joined with the physical god of the earth and in that union they produced the life god which was a partnership of the two and in the image of the Creator which was the union of the two. Then the life god, seeing his domain multiply by cloning, said: "I shall separate my domain in the image of the two who came together to form Me" and life separated into male and female and the life god said: "It is good. Because the female is like the physical god which nurtures the physcal body. And the male is like the abstract god which enters the physical body and rejoices. Thus, life will recognize its orgin." "C'est la vie!" The life god soon lamented, "in life the two again are one but life accepts its existence without question or thought of purpose."
Solemnly the life god approached the animal of the highest order and whispered into its ear. And the animal realized its mortality and became man. Knowing death, he was able to perceive life and then went on to define all which he perceived in life as pairs of opposites. And man refined language so that he might confer with others in seeking the whisper's meaning.
And man invented time and developed tools to quicken his labors and allow more time to be allotted for comtemplating the wisper's meaning. And man developed law and community so that he need not fear while pondering the whisper's meaning.
And "good" is that which enhances man's quest for the whisper's meaning. And "bad" is that which delays or distracts man's quest. And "good" comes from the way man responds to fear and desire. And so does "bad."
And as the life god whispered, in sacrifice, he entered man. And this sacrifice was foreseen to happen before the One first shouted. And before the conception of man, was man's purpose conceived. And before that was the Void, which was the One. And the words of the life god's whisper are: "It is I."

A perfect example of 'And man created god in his own image'.