Disappointment and Optimism are two sides of the same coin. Disappointment is a result of expectation not being met and expectations are a result of positivity.
Children cry often because they are optimistic. They have a sense of loss when they don't get something. As we grow older this unbounded optimism dies; we just call it becoming an adult.
There are tons of books out there talking about positivity and the need to be positive for good things to happen. While that may be true, being positive comes with its share of being disappointed.
Most of us give up. The first failure; the first loss; the first heartbreak. Each of these takes a little bit of positivity out of us. The challenge as you are growing up is not to.