Much of our world is subjective by nature and our thoughts lead to actions and outcomes that have a significant impact on our personal level of happiness. One could theoretically argue that happiness is truly based in our thoughts and for the most part is not dependent on money or circumstance. Others might take issue with this idea but if we think of how interpretation creates our own realities then we might be surprised by their outcome.
Consider an event that could potentially be seen as negative such as a difficult project or a personal set back. While these events do produce short-term stress much of it is based on how we view and interpret these issues. That stress can quickly reside if we understand the situation completely.
Thoughts such as "this sucks I can't do this" or "there isn't enough time and I'm going to fail" will produce more stress and in turn lower your happiness level. On the other hand, thoughts such as, "this frustration will be short-term but the knowledge I gain will be long-term" or "I can beat this and master new skills" will lead to feelings of empowerment.
Thinking positively doesn't mean we should be dishonest with ourselves. It means we recognize the problem and challenge and make positive steps to change it constructively. If it is a pattern of problems then we might consider making long-term pattern changes in our lives. A positive outlook is about believing in yourself and your ability to overcome challenges.