Society

Humans are inherently selfish. Society and our parents teach us to stop grasping and demanding as we grow up, but babies, in our most innocent and natural state, want to be catered on hand and foot, fed and changed and cradled with nothing in return whenever they demand it. (Partially this is because they're so dependent: our young are born extraordinarily incapable of taking care of themselves, and extremely slow to mature. There are evolutionary advantages to this, including a larger, better-developed brain, and a long life span leads to a long adolescence. Elephants have a lifespan comparable to that of a human, and reach sexual maturity, a historical marker of adulthood in human cultures around the world, at ages nine to seventeen.) (Information taken from here at animalinfo.org.)

So society serves a purpose, because no responsible parent lets their child grow up selfish. As the brain develops and it becomes possible for the child to grasp concepts like manners and responsibility and selflessness, parents begin to teach their children not to steal, how to share and when to say please and thank you. Of course, this isn't a perfect system, and there's always exceptions, for numerous reasons: not all cultures even share the same values to be taught, to start with, and there are bad parents or neglectful ones, and children with social, developmental or psychological disorders who can't grasp the concepts or apply them. The process is a long one, too, extending through childhood and into adulthood--and the brain isn't even fully developed until between the ages twenty-five and twenty-seven, long past when most people consider themselves adolescents.

This is the point and purpose behind manners. It's not a matter of which fork to use (start on the outside and work inwards with each course) but instead a system of rules and regulations to provide clarity to social interactions, and to help combat instinctual selfishness. You say thank-you because it's polite, not because it's instinctual and natural, and that simple phrase does a lot to prevent resentment. It keeps the wheels of society spinning smoothly.