I don't know why morality would be absolute for an entire society.

Morality might be absolute if it came about through evolution, as Arch suggests above.

I also don't know why they would be relative for an individual.

Nietzche (sp?) said that if there is no god to represent absolute values, then the result has to be values that each of us works out for ourselves, i.e. relative values.

(I'm not sure I agree with Nietzche; perhaps I need to read more Jung, Plato, etc.)