Behind it all is Ego.
Over-generalization is a flaw of logic. Some people are not intelligent enough to understand the requirements of logic, so we need to be tolerant with them. But when those same people are intolerant, then they are letting ego get the best of them. Even the most intelligent of us can use over-generalizations and the force behind it is ego.
Ego attacks the lowest as well as the highest intellects. It presents itself as arrogance. Humility is the antidote.
Forcing beliefs on others is also a function of Ego, because Ego always sees itself as right.
Ego is the false self. It is separation and self-concern. When we love each other as ourselves, then we give up ego. An atheist can never understand this, because the "self" that is left after ego is gone is spiritual. This goes beyond physical "compassion" to pure altruism which has no self-concern, because it exists outside of space-time, energy-mass.
And there is nothing "hardcore" about spirit. It is the lightest of "efforts" and the softest of "beliefs." It is pure knowing that is effortless.