The meaning and origin of the expression: Men's evil manners live in brass; their virtues we write in water

What's the meaning of the phrase 'Men's evil manners live in brass; their virtues we write in water'?

This Shakespearian saying expresses the notion that, while we recall well anything done to harm us, we forget quickly the good others do.

A modern phrase that expresses a similaridea is Monty Python's "What have the Romans ever done for us?". The characters in that sketch claim that "The Romans have taken everything from us" and offered nothing in return except aqueducts, sanitation, roads, irrigation, medicine, education, wine, public baths and peace.