How can one not believe the Holocaust happened?

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I definitely DO think it happened. My grandfather on the other hand, did not. I always had to wonder why or how he could deny this, as his daughter married a Jewish man, whose mother was in a concentration camp as a little girl, and had the numbers tattooed on her arm. I have listened to and read about the conspiracy theorists, but it just doesn't make sense to me. How can something like that on such a grand scale, have been faked? I mean, if you were living in America at the time (or anywhere else removed enough, where you really had no clue what was going on), it was probably easy (at least at first) to come the conclusion that it wasn't true. But if you lived in Europe, how else did you explain the mass amounts of people disappearing from existence? You can go to Birkenau and Auschwitz, which are now memorials. You may have met somebody and seen their tattoo. You may have been to the Anne Frank house. Can one honestly say with a straight face that this didn't happen?