Radiation and chemo therapies are one of the most horrific things one can endure.
Depending on the type of cancer, you can go in a few weeks, months, or fight the battle for years.
Even if you endure and persist, which gets progressively easier for a lot of cancers thanks to more advanced therapies, you have to fear recurrence.
I'm not at all opposed to ending one s own life if done without severe psychological issues and done thoughtfully after plenty consideration.
Of course, if you regulate assisted suicide, as some countries in Europe have done, it leads down a very slippery slope. Who makes the decision on whether you are allowed to end your own life?
Most current religions frown upon suicide as a sin. Life is god given and not yours to decide upon (Islam, Christianity). Or suicide is viewed as an unjustified interference in the natural order (Hinduism). And therefore most of our societies reject it.
Admittedly, most suicides that are not connected to terminal illness manifest due to psychological / neurological illness and despair that might be cured or managed with proper therapy and medication, making a return to a happy life possible.
It's a fascinating and very multi-facetted issue to think about.