At the risk of sounding extremely sappy and twelve, I have to admit that after a full day of pondering this question I have come to the conclusion that I believe (at least at this point in my life and with a not-so-impressive maturity level...) that all we need is love. Love is the one thing that ALL people (no matter their social standing, income level, cultural background, age, etc.) strive for. Be it from one's family, friends, romantic interest, or God, everyone wants to feel needed and cherished; being truly happy and content relies on one satisfying this craving. With love, the poor feel rich, the rich feel equal, the powerful are humbled, the insignificant gain meaning. To survive, yes we need basic bodily substances, but surviving is not truly living. Material possessions, also, to not allow us to live. Love, though, makes true living possible. It is therefore the one thing that we truly need.
*end rant*