Thoughts and feelings may be dormant whilst you are awake, or you might just be too pre-occupied during the day to notice such thoughts. Then, when you relax during night time hours, the brain remains active, and these thoughts can be transmitted into your dreams.
There are a few scientific theories associated with what causes certain dreams but, although there are many different assumptions made, we really don't know exactly what triggers them.
A popular view of dreaming suggests that the causes of bad dreams lie particularly in the need to release stresses from the day. Sigmund Freud believed that repressed or suppressed fears were the causes of dreams, particularly bad ones, and that they could be stopped using psychotherapy.
Traumatic life experiences can often be a key factor in the causes of some dreams that are related to the memory of the event.
Having the same dream over and over again would suggest a sub-conscious worry, or tension. Maybe something that is on your mind, but that you are not aware of, is troubling you.
Try to relax as much as possible before going to sleep. Take a hot bath, or have a hot drink -anything that might cause you to have a deeper and more regulated sleep pattern.
If the dreams persist, it might be worth talking to a hypnotherapist who will be able to offer some advice on the problem.