Because we are all One. Everything and everyone is interconnected so when we treat others with less than our best we are actually treating ourselves that way. This in fact is why there is so much power in giving - we actually receive more.When someone else's, less than perfect behavior, triggers us, it is usually some shadow we have disowned in ourselves - so instead of judging the other person we should in fact look within ourselves and see what caused the trigger. This in fact makes it easier to empathize with the other person as you realize they are coming from a place of not feeling good. I think it stands the test of time because at the core of our being we know that we are all connected so when we hurt others we are in fact hurting ourselves.

This is a universal rule actually, it is in every religion and philosophy , http://www.religioustolerance.org/reciproc.htm , there is actually a rule in marketing called the Platinum Rule. The Platinum Rule is "Treat others as they would have you treat them." To understand this you would never treat say the Queen of England as you would have her treat you, she would expect to be treated better than that.

because you would wish no ill will unto yourself would you

I would interprete this rule as another way of describing the Law of Attraction which says that 'Like Attracts Like'.

We get things that are vibrationally similar to the dominant vibration we emit. Good attracts good, bad attracts bad.