Why do people deny the holocaust ?

I think it comes from people who feel like the tragedy was overblown. Like, they won't deny that something happened, but they DO deny that that many people were killed. These are simply people who want to decide facts for themselves, rather than allow someone else to decide the facts for them. Since those people couldn't be there to affirm the facts in person, they have doubts of the motives of those in charge of releasing those facts, and they get attention because those doubts are so outlandish.