What is the inherent difference in people that some require proof and others need no proof of God?

I know Jesus said that they hearing hear not and seeing see not lest they see with their eyes and hear with their ears, because their hearts are waxed gross, and they seeing, see not and hearing hear not...what do you think waxed gross means here? I feel I have an inner "knowing" and I have needed nothing more, but for some who do, why is that? and for those who don't why don't you? Thanks for your imput.