We don’t know. But the following theory seems to be the best (it can be falsified however one day).
All cells of the body have certain life span of some days (white blood cells) to years (nerves). After this time new cells have to be created. The only way is cell division. Two cells remain, but only one must survive, other ways a tumour will proliferate. For this reason cells have a mechanism called apoptosis, a kind of suicide. From specific organelles, mitochondria, a messenger substance is secreted and apoptosis starts. Sometimes the outer membranes of mitochondria are tight and the cell cannot perish.
We will have to look on mitochondria now: They all originate from the mother only and have an own genetic substance. They reproduce by cleaving. Briefly, they behave like autonomous bacteria living in the cell. Many scientists think that in an early state in evolution they had been bacteria that enabled higher cells to use oxygen for respiration. They are sharing another feature with bacteria. If exposed to toxic substances they close their membranes. All mechanisms sealing a membrane are therefore likely to cause cancer as well. A sealed membrane however means, a cell can no longer use oxygen for respiration and has to turn to fermentation. This exactly is what happens in cancer cells.
Therefore in cancer one shall always look, which toxic substances could be the reason. It may be mercury from amalgam fillings in teeth or bacterial poison in incomplete filled dental roots, fluorine in toothpaste, impregnators and preservatives in textiles from low-wage countries (where many things are still used, long forbidden in developed countries) but also rays from mobile phones, PC-screens etc. If you have got a real enemy, make your micro-wave a present to him.
A very important reason (according to estimates up to 80%) is continuous stress. By stress adrenalin is poured out, until postproduction is disturbed. In cancer patients adrenalin levels are very low (Hüther 1998).
In this case, insulin as its antagonist becomes active and causes a transport of sugar into cells. From a specific concentration sugar inhibits mitochondria. Continuous stress is an important reason for cancer therefore. It is very well known for long time that tumours occur after a shock of not coped mental or social conflict.
Another fact is important: In psychotherapy sometimes a method is used that follows the method of a lie detector. If somebody answers the question: „Do you like your mother?” with “yes” and the test shows “lie”, the therapist knows where to go next. Normally it is no real lie but supersession. In many cancer patients answering the question: “Do you like to die?” with “no”, the test shows “lie” (Reitz 2013). People commit suicide often because they do not find a way out of a conflict they consider as vain. Suicide seems to be the only avenue of escape. It seems that in many cases of cancer such a hidden death-wish exists.
The body is controlled by the spirit: Therefore it makes little sense to fight against cancer without solving the conflict in the back ground. A tumour being removed will either return or another deadly disease will occur. Without help most people will never escape from the situation. It is good to remember that not all patients really want to become healthy. Asking patients they very often know who is guilty of their cancer, the “evil” floor man or the “evil” mother-in-law. Frequently these people are only “evil” in in the head of the patient. For the person concerned however, it seems to be like a voodoo-spell or an evil eye causing the disease.