Astonishingly, in spite of decades of research, there is no agreed theory of cancer, no explanation for why, inside almost all healthy cells, there lurks a highly efficient cancer subroutine that can be activated by a variety of agents - radiation, chemicals, inflammation and infection.
Treatments and therapies have never been better in the face of this disease. You have every reason to be optimistic, determined, and focused on the future. My only other advice is to cherish every moment with those you love at every stage of your journey, as I have done this summer.