Well, dreams are dealing with memories. The activation of memories is triggered by special events which are similar to those that are associated with these memories...
So, imagine there are memories you can´t reactivate, things that are so important for your life, for your way you see the world and what you are that you want to reactivate them. In dreams it is possible, but dreams are most often "encrypted" in your own unique language. Lucid dreaming gives you the possibility to understand what is going on in your dreams and gives you the chance to see clearly, maybe faces or places...
You can´t control what you are dreaming but the dream and through this the emotions in your dream aren´t controlling you...