What does it mean when you dream your boyfriend leaves you?

It may mean that you are insecure in your relationship and are not certain about his level of commitment. It may also just be a sign of general anxiety manifesting itself as a dream scenario where something unpleasant is happening. In some cases it could be a symptom of the dreamer wanting to be free of the constraints and obligations inherent in any romantic relationship.

In isolation from other information about the person, it is not wise to try to assign a particular meaning to his/her dreams. That's part of why psychiatrists, psychologists, and counselors discuss not only a person's dreams, but all the things going on in their lives.