From the Art of Worldly Wisdom -- a collection of aphorisms from the works of Baltasar Gracian, as translated by Martin Fischer.
#19. Do not enter where too much is anticipated. It is the misfortune of the over celebrated, that they cannot measure up to the excessive expectations of them afterwards: never can the actual attain the imagined: for to think perfection is easy, but to incarnate it, most difficult...
Too often, we avoid being the "bad guy" by not saying no, or by not correcting people's lofty expectations. We don't give ourselves the opportunity to sweeten our relationships by setting the bar low enough that we can do more than what's expected.