As we grow older, we tend to drift from what we believe and our hearts grow colder and colder and harder and harder.
I would think because we come to the rationalization, for whatever reason(s), there is no god.
But if there was no God, then why does our heart says there is a god? Why do we tell our heart to shut up
I would assume it's because our belief in god came from those whose actions come with best intentions for us. Especially if instilled from one's parents. Heart here being implied as our inner being of love and trust, as children that comes with a blind faith as they are incapable to distinguish otherwise, that can be seen with studies done with abused children attachment to their abusers. Individuals are driven by what they come to know, their level of trust is based on that. That changes over the course of one's lifetime.
RE: DEISM = ATHEISM