You can see it in everything he attempts to do around the globe, every conflict he wades into, every crisis he refuses to concede. And as John Kerry prepares to step down as secretary of state next month, he will carry it with him just as he has for 50 years: a deep-seated belief that America -- and, indeed, he -- can solve some of the world's thorniest problems with the right mix of politics, diplomacy, perseverance and personal charm. For better and worse, Kerry never gives up, even when everyone around him thinks he should.He is an indefatigable optimist, a warrior -- occasionally quixotic, always gung ho -- who refuses to stop until he's made every last phone call, bent every last ear, appealed to every last world leader who might help him in his quest.Interviews with Kerry and more than two dozen US and foreign diplomats, State Department aides, current and former senators and foreign policy experts over the course of eight months paint a pi...