You are going to get all sorts of answers to this question. My answer is NO_not every dream is to do with past lives. They do have relevance to your state of consciousness which is a topic that needs a wide berth because not everything you are conscious of has rational meaning. Our minds have this overpowering urge to make sense of absolutely everything it sees and so any visions that come in dreams will be placed in a category.
Even in regression therapy, not everything is a past life and it's not that black and white to determine what is and what is not a past life. Experientially there is major clarity on what is a past life.
There are dreams I have had that are of a prophetic nature. The scene played out was not one you would see in human reality. But the message certainly was there and it came true. It was not of death, ie someone is going to die, more a warning or playing out of which direction my current life was taking and to be aware that it was not a positive scene. Hindsight is 20/20 as they say. I didn't understand that dream until it was actually playing out!
There are a plethora of experiences in astral worlds (which includes the sleep state) and to say it is about past lives is simplistic. A good rule of thumb when interpreting dreams is to ask "what does this mean to me, and how is it relevant to my life now?"