People automatically have multiple personalities. The reason we don't see it in each other is that we all have a central authority in us that combines our multiple personalities into one personality. If the central authority in us, or the pathways that send commands to the various personalities in us, happen to become disrupted for some reason, the multiple personalities can express themselves individually at different times, in very emphatic ways.
One of the strongest examples of multiple personalities expressing themselves in people is in people who have a mild to medium stroke. A stroke disrupts the central authority a person has in himself, and/or the control pathways to the different personalities. Many people can tell you stories of friends or loved ones who, after having a stroke, jumped into different aspects of their lives with great vigor and zeal. For example. Sometimes they are very emotional talking about childhood. Sometimes the become wildly angry about politics. Other times they might be more loving than ever towards some good friend or their spouse. Other times they are angry with the same friends, and can't be talked into a calm mood.
Check out yourself for your different personalities. At times you might be concentrating on a project. Other times you might be relaxed in a good game on TV. Still other times you might be in an almost over-compelling desire to make love. But if you are normal and healthy, you can talk yourself out of these moods, which are really various personalities that are living together within you.