Determining why you are running is an important step that should be addressed early in a campaign. You should have a clear definition of why you are running for office. It must be concise and well verbalized as voters will ask.
In the beginning, it may in fact be a very simple reason. Later in the campaign, you should expand and define your top 4 issues of why a person should vote for you.
In many cases, the reason you are running will dictate your message and strategy up until election day.