Deny, Deny, Deny

Denial: to reject something you know to be true. Why do we do it, if we know that it is true or at least some part of us knows it's true then why bother denying it? Someone has been my friend for a long time but recently I am faced with the knowledge of something that both myself and this person have been denying. I chose not to believe the evidence saying it was pure speculation that brought myself and others to this certain conclusion but I don't think I can deny it any longer. My only question is how long can they keep it up? To deny oneself of who they are must be tiring.