Prometheus mirrors the ongoing theme that cuts consistently through Lovecraft's work: the complete irrelevance of mankind in the face of the true powerhouses of the Universe, in this case the Engineers. Just as David has been made to serve frail humankinds own self-interested purposes, so have we been made to fulfill an as yet largely unspecified role for the Engineers. In the Prometheus universe there is nothing special about humankind when we look at events on the cosmic scale.
The movie to some extent accuses mankind of believing that the Universe owes it a living and an explanation for its place in the cosmos. Nowhere is this better conveyed than in Peter Weylands all-consuming self-centred and self-satisfied desire to obtain the secrets of eternal life from the Engineers. We now know what David says to the Engineer as he acts as the mouth of Mr Weyland (fittingly, man alone could never have decoded the key to the Engineers speech, it took the efforts of his manservant- alone for 2 years on board the Prometheus- to decipher the Engineers language)
This man is here because he does not want to die. He believes you can give him more life.
As far as cosmic introductions to our gods go, its hardly friendly and its hardly reverent. And ironically the clue as to how the Engineers might react to this request was there all along. It was embodied in Holloways condescending and discourteous treatment of David, which hauntingly foreshadowed the way in which our own creators might choose to treat us: at best with condescension and ambivalence, and at worst with, well... a murderous rampage.
Prometheus suggests that no matter how hard we look for answers we will meet futility. This is best communicated by the vision of the Prometheus as it enters the barren desert of LV-223, the desert itself a metaphor for the barren universe. Davids statement that no man needs nothing suggests that man is by his very nature inclined to question, but there are no meaningful answers out there. Theres nothing, says Peter Weyland at the end of the movie, as he is poised halfway between life and death in the darkest region of the cosmos. I know sir, have a good journey says David. David, with his matter-of-fact vastly superior computerised intellect, has long ago calculated the improbability of there being anything beyond corporeal reality. In the words of Chuck Palahniuk "this is your life, and its ending one minute at a time"
Key line: David - There is nothing in the desert, and no man needs nothing