Cancers are produced by cells that have gone mad. Normal cells, on the other hand, are the sanest things in the world. They are polite and enjoy the company of other cells. As a community, they work together for the good of the many, are well organized, hardworking, and content with their lot in life. Heart cells enjoy pumping blood, brain cells would not change places with skin cells for anything under the Sun, and no way would any of them do anything that would jeopardize the health of the body they so carefully constructed. Cancer cells do not care about any of this. They do what they want and go where they please. If they decide to build a large tumor in the middle of the brain, well, that is what they are going to do, even if it kills the body they are living in.