My CFer has not asked this question yet. I'm not sure when or if he will reach the cognitive ability to ask this question. But, his older brother has severe Hemophilia and has asked many times, especially since he recieved no treatment for his Hemophilia in his birth country and so coming to his forever home also meant coming to weekly needles, treatments, assessments for bleeding, etc.
Anyway, the simple answer I give is that he was simply born with it. I do give a bit more when he presses which is highly religious and in line with our faith. But, the long and short of it is that this world is not perfect and none of us are perfect either. Hemophilia is something he was born with, just as he was born with thick curly black hair and his brother was born with his W. African facial features.
The answer will be the same for my CF child if and when he is able to ask this question (and I pray he reaches the cognitive ability to ask this question someday). Sometimes we simply can't understand why. It just is what it is. Yeah, treatments and needles and other things suck. But, if they didn't have their disease they wouldn't be them, and there are a lot of GREAT things about them I wouldn't want to miss out on too.