Your mind is a kind of dream generating machine. The thoughts, images, and sometimes even emotions are all aspects of dreams. When your mind is really going it can take you for a ride, much like it does at night when your body is asleep.
How can you tell that what your mind generates is a dream? How can you be sure that you are not asleep in your bed right now experiencing this all as a dream? If it were a dream how could you be able to tell?