I didn't read all the responses, so forgive me if I'm repeating. Quite frankly, I think it's ignorance. People don't know much of anything about CF unless they are medically educated (and even then, CF knowledge is questionable in some), or unless it affects them personally somehow.
When someone says "you're so strong" or any variant thereof, I always respond with, "Well, it's all I've ever known. Sh*t happens. You think it's hard because it's unusual to you, but if you were born with it, you'd do as well as me" etc etc.
Keep in mind also when you say CF, and explain anything about it... for one, it "sounds serious." Cystic fibrosis sounds kind of scary and evil. It is, but hell. For those who DO know anything about it, usually they know the brief dim parts... life expectancy, etc. So it's a shocker, and people <i>never</i> know how to respond in those situations. I always try to explain with a laugh, or make a joke, so it calms them down. Generally, if you explain it lightheartedly (but be truthful, of course), they see you can handle it. Therefore they are better able to handle responding. It's kind of like they take cues from you. If you get quiet and don't know what to say, they won't either.
As someone else said, it's often out of lack for something better to say. Same reason we often get "Oh, I'm sorry" Or "Oh, that really sucks". It's just all they can come up with. And that's okay. Karen, I believe, also mentioned, they think it's a compliment. So as much as we may find it annoying, keep the intentions in mind, because it's generally meant well. Kind of like little old ladies who offer you cough drops. They may be annoying, because we get it often, but you smile and say no thank you, maybe give a brief explanation, maybe not.
