There is a human instinct to talk about things higher than ourselves. This was instilled in us because we are meant to speak about Hashem and His wonders. But a person has free choice as to how to utilize this instinct. Some use it to deny G-d. The incredibly strong instinct that atheists and agnostics have to deny or doubt G-d comes from the same place from which others praise G-d. The latter used their free choice for the good while the former used it for bad.
Others utilize this instinct to talk about Gedolim and Tzadikim in a positive and admiring fashion while those who choose evil use this same need to defame the Gedolim and Tzadikim.
[עי' בזה ב'רזי לי' עמ' נ"א-נ"ב]
