Sincerity is rare

Not because people aren't sincere, but because being sincere means feeling things fully and sticking to it - sort of like how a child would be

creating and destroying; flying cars, roc.camera, july.rocks

Let me add something into the mix: sincerity, or honesty, is something that only a few people can afford. Why? By being sincere or honest, you sometimes risk your position, and you can lose what you have. Thus, I think being sincere is sometimes related to your power, and that's why people are not sincere nowadays; people can lose too much.