Jesus had a reason when He counseled us through His Word to "not let the sun go down on our anger." (Ephesians 4:26) One reason is that our emotions have a huge effect on our immune system.
We are exposed to illness and cancer-causing factors every day. Our environment is literally toxic.
If you research carcinogens in our world the list is endless: flouride in our water, sugar, preservatives, pesticides, plastics, cosmetics, cleaners, etc, etc, etc. The one that always makes me shake my head is that one of the common ingredients in sun blocks is a known carcinogen.
After considering this list, it's more amazing that we ALL don't have cancer. Actually, our body has cancer cells in it right now. The difference between those that "get cancer" and those that don't is our immune system.