There is much truth in humor, but none of it seems to be usable. How many times I have heard a comic state something that strikes at the heart of a feeling only to tuck it back inside quickly. Perhaps it is simply too truthful or perhaps it is a statement reflecting something that is innately within and is therefore worrisome. Those dark thoughts are an ingrained behavior which we know we will not change.
Are there thoughts and behaviors which we cannot change? Are these thoughts and behaviors difficult to change because they have biological interactions as well? Certainly many of these behaviors are known as a vice, and certainly, the root of vice lives within the biological interactions of man with his surrounding world. There are no vices which are not manifested in the physical world.
In this perhaps is the lesson, that vice is the foundation of dark thoughts. If so, then good thoughts are the only thing that can bring the house of vice down, as it must certainly be shaken from its foundation.
Once again we see the battle that matters is between your soul (the true self), and your interaction with your mind (the often master-less servant). I believe I have seen these two entities before in cartoons and film. An angel and a devil perched on the shoulders arguing between vice and virtue.
May you choose virtue more than vice, as your history is the sum of this decision.