Everyone does have mood swings, but they are usually much less severe than in bipolar. Most peoples' mood swings are a direct response from changes in the environment. An argument with a friend, a rough week at school or work, and so on. Bipolar is non-environmentally affected - environmental changes can only amplify what is already there. And that is why it's incredibly hard for me to answer the dreaded "what happened?" question, when I'm depressed.