Thoughts create moods which influence behavior and finally express themselves directly and/or indirectly as physical reactions. The body is connected to the mind and they communicate intimately with each other.
Just thinking positively may distract us and make us avoid the more important signals our bodies are trying to tell us: hey, something is wrong!
Let's say you have a problem and it is so frustrating that it is making you feel mad, letdown, depressed, irritable. By considering the situation from as many angles as possible, negative and neutral as well as positive, your thoughts are reformed to view things from a new perspective.
This reprocessing of mind can lead to new conclusions and solutions that make you realize that things aren't as bad as you thought, pushing depressing thoughts away and opening you for healing through change and improvement.
Mind over mood: the book and the website.