Personally, and from experience, it comes partly from empathy and a lack of connection with whoever has told them it, and an unspoken embarrassment as in 'I don't know where to place myself' ......
I've seen it with folk over my mater and her Alzheimer's - one woman telling me that she has been cured of it and there was hope, my cynical reply being 'well I hate to tell you that it is incurable at the moment - unless you are related to Ernest Saunders, he of Guinness fame, who claimed that he had the big A to prevent him going to trial - and then there was a miracle, as he's the only one sufferer that has been cured to date.'
I should have put this is my forthcoming book - damn..... 19 says to launch online....