A good one from Barbara Crafton:

"It's hard to come back to a place of trust once you've ventured into the place of sarcasm, isn't it? Once you've activated your cynicism, it bleeds quickly into every other part of you, including your heart, which promptly snaps shut. You don't want to be caught trusting in anything too completely. Don't want to look like a fool. Hah! you say, dripping derision from the single syllable. Hah! you say again. Oh, puh-leeze! you say, trying that one on for size. And it fits: you feel a little better about your hopeless situation, a little more in control. At least you still have your sophistication. "