Dear OP - we've known many people who are happy and cheerful people and also have cancer. i got hy dh in to answer you properly as he knows much more than me about the science:
This is lurker's husband here, BSc Biology, with ten years' experience in medical imaging and a mother who was a cancer nurse:
Cancer is caused by (usually many) mutations in a cell's DNA which cause a malfunction in the mechanisms that regulate the cell's growth.
It's important to distinguish between the two different types of agents which have a causal role in cancer. Cancer biologists term these "initiators" and "promoters"
"Initiators" are the nasty ones ; honest-to-God carcinogens. These are things that directly and irreversibly monkey around with DNA and their effects are dose-dependent. Things like X-rays, ultraviolet radiation, or chemicals like vinyl chloride (the building block of PVC) which are safe enough in small doses but which you want to avoid a large encounter with.
The vast majority of "could cause cancer" agents fall into the second category of "promoters". Almost anything which appears on the front page of the Daily Mail comes into this category. These are things which do not cause cancer on their own, do not directly damage DNA but when combined often enough with initiators which do damage DNA, could cause cancer.
Their effects are much harder to understand, they're generally not dose dependent, multiple factors are at work so it's hard for any epidemiologist to filter out the "noise" you get when doing studies on them.
Into this category falls things like saccharin, aspartame, our own stress hormones cortisol and adrenaline, and our own sex hormones testosterone and oestrogen.
Lifestyle does play a factor and we know this because immigrant populations from one culture who migrate into another change their prevalence of different cancer types.
None of this is a consolation when things do go wrong
These "natural health" magazines are filled with claims which are usually totally unsubstantiated, never cite any form of learned research done under controlled conditions and should be exposed in the daylight for the nasty, deceitful, scaremongering crap they are. Pay no attention to them.
And to those who claim that "natural" is somehow better than medicinal chemotherapy, it is worth pointing out that the natural world has given us almost all our medicines.
Most of the cancer drugs in the arsenal today are natural products too and most of them are directly from plants or slightly modified versions of plant products. For example, vincristine comes from the rosy periwinkle, tamoxifen derives from an extract of the humble yew tree.