Nobody is downvoting "the fact that doing something to yourself is not comparable to cancer": that observation is routinely made on this sub a hundred times a day. You're being downvoted for misreading a simple analogy about how body positivity can entail recognizing your ill-health and working to change that ill-health while still loving yourself and learning to be confident in your body. That was the limit of the analogy between obesity and cancer and how body positivity can function in both cases, but you ascribed a lot of things to this user's comment that were neither said nor implied --- and you came in with both guns blazing in the process. As I said upthread, misreading and misrepresenting someone's comment and then reacting with hostility to that user based on nothing more than your misreading aren't generally well-regarded: that's why you're being downvoted.