In Dark Souls, when you first start off you have a fairly high chance of getting Soft Humanity whenever you kill an enemy. As a fairly experienced player, I rarely die before I ring the first Bell of Awakening, so by the time I have reached the Gargoyles, I already have enough Humanity to Revert to Human, and Kindle the Bonfire.
However, as the game progresses, regardless of how well I might be doing, collecting the Liquid Humanity from fallen enemies becomes less and less of an occurrence.
What affects this?
Soft Humanity: the Humanity that goes straight to your counter when you collect souls after killing an enemy; characterised by a black stream