I have tried to imitate "The Face" here:
I think that this happens because, despite the great numbers of people with cancer today, we as a society, have not kept up with creating standard appropriate and helpful social responses to people who have really scary diseases. As a result, you get things like The Face or (worse) weird, nonsensical, unhelpful, sometimes-insensitive comments / suggestions/ wisdoms.
Sometimes it's easy to get a little honked off when, after revealing to friends that you have cancer, you receive a response like, "Yeah. My dog died of cancer. Cancer sucks," or "My uncle died of that kind of cancer. He got cancer and was dead in two months." But I think that, at least after a while, most of us realize that these comments (and The Face) come from well-meaning people who are uncomfortable about illness and ignorant about how to respond. Also, there's probably some self-importance in here, but I digress.
The best advice that I can give to someone who is at a loss about how to respond when someone with a cancer diagnosis is keep it simple. Don't offer diet advice or talk about alternative treatments in Mexico. Don't share cancer stories that had bad endings - or happy endings for that matter. Also, don't avoid the subject. Try to imaging how you would want someone to respond to you if you were the one with the diagnosis. Comments like, "I heard about your diagnosis. I was so sorry to hear about it. How are you feeling?" "Please let me know how I can help" (only offer this if you mean it). "I'm thinking/praying about/for you." These are simple and kind statements that focus on the ill person instead of any advice or cleverness, and lets them know simply that you care. In my experience, this goes a long way and gives significant comfort and some relief.
“Or, rather, let us be more simple and less vain.” ― Jean-Jacques Rousseau