There are so many different theories about what cancer is. The conventional theory is our own cells get out of control (proliferating) and stop dying like normal cells would after a given time period. (Different body cells have differing life-spans and the natural death of a cell is called apoptosis). They carry on dividing and building, growing into new cells that also don’t die. This theory doesn’t however tell us why this happens. They can’t seem to explain why the cell does this, what causes it or what will stop it.
Nature does not show a propensity to attack it’s own organism. There is no proof anywhere that this would happen so why would we believe it of our own bodies which are perfectly designed for survival in their particular environment. Do things go wrong when the environment is compromised or has nature put things in place to help us overcome and survive possible problems. From all of nature we see proof of the latter. Therefore cancer MUST be a survival mechanism.