This is only tangentially related, but, when I came out to my (very moderately religious) parents as an atheist, my father got a little ticked off when I would sit silently during grace before dinner, rather than reciting the prayer. He said it seemed as though I wasn't thankful. My response was something like: "I am lucky to have been born in a country where not many go hungry, I am lucky to have been born to a family in that country that never goes hungry, and I am appreciative to you for cooking the meal, and to the workers on the farms who cultivated the food. However, the prayer we say is not directed at them, nor could they hear us if we prayed to them. I am not unaware of how lucky I am, I just don't think that this luck was orchestrated by a deity."

Personally, I think thanking god for survival of cancer is silly for the same reason: generally, it's the result of a lot of very hard working (or at least, very educated) people. If God cured your cancer, he also gave it to you ,etc. It's just a weird construct. I think I understand why people feel uncomfortable expressing gratitude towards blind luck? Maybe you feel lucky to have survived cancer (even if you worked hard to find doctors who could help you), and it doesn't feel like enough to just thank those people? It might also be a way of dealing with a sort of "survivor's guilt", you lived, but not all of the people at the treatment centers did, why? Luck, or chance aren't as satisfying to people, I think, because if you credit them, it's like you're admitting there's no over-arching logic to why you survived. No sentient being liked you more, or has a plan for you. Your body just responded better to the same type of treatment as everyone else. And, you have to supply your own interpretation of that.

Okay, I think I'm done speculating wildly now.