A good friend of mine is absolutely one of the most positive people I've ever met. Every time I've ever called him and asked how he was doing he has answered, "Fantastic!" If I feel myself slipping I'll call him just to get a shot of positive.
I once asked him about his perspective and he said, "You've got to be positive. If you don't the negative stuff can overwhelm you." Simple.
My father and mother used to have a banter that went something like this when my father got up grumpy: My mother would say, "Bill, as soon as your feet hit the floor why don't you just say, 'I feel great and it's going to be a wonderful day!'? If you do that, it will be." To which my father would laugh and answer, "Marie, why get up and lie to yourself first thing in the morning?"
The key is that you have a choice. Whether you believe that the day will be great or not is your choice. Hoping it will be and being surprised is better than being sure it won't be and being proved right.
My parents had a friend who joked that the first thing he did in the morning was read the obituaries. He said that if he, himself, was not in them it was a good day.
Being vertical and maintaining a steady respiration are good enough reasons to be positive.