As a cancer survivor, why would feel sad?

Survivor's guilt: Many survivors feel guilty and that may cause sadness and depression. What ever may be the cause, you should consult your doctor as effective treatments in the form of psychotherapy and medications are available to address the issue.

Fear of the Unknown: From my experience as a cancer surgeon, the two most difficult times are immediately after treatment, when one feels as if nothing is being done to treat the cancer (often after months of rx). The second most difficult issue to overcome is the fear that the cancer may come back even if all systems are clear. Of course, there is no simple answer, but with each passing month, this improves.