I would never suggest that positive thinking is the one and only panacea for all of life's problems. However, approaching your life from a place of optimism is likely to keep you healthier and happier.
Humor can be one way to break up a negative thinking pattern. Read books or watch films that make you laugh. Go see some live comedy. It's hard to be negative when you are laughing.
Definitely true. All of my happiest moments involve laughing so hard you have to spit out the drink that's in your mouth or end up snorting it through your nose. There's a lightness and joy in laughter that helps to take you out of an unpleasant situation and really see the world in a larger perspective. Laughter is definitely one of the best medicines I can think of.