According to Buddha, the root causes of our suffering are anger, attachment, and ignorance. I have also heard these things referred to as hatred, greed, and delusion, respectively. According to the Buddha, we are attached to everything around us in life, including possessions, people, places, concepts, feelings, and desires. Because nothing in life is permanent, we suffer when things change or are not exactly the way we would like them. As for anger, we know that all our actions have consequences. Do good, and it comes back. Do harm, and the same is true. Most of our actions that cause harm to others are because of our own anger. For ignorance, we remain in cyclic existence because we do not realize the inherent emptiness and impermanence of all things. After defining anger, attachment, and ignorance as the causes of suffering in the second of the Four Noble Truths, Buddha also advised in the Fourth Noble Truth that in order to end all suffering we must rid ourselves of these three poisons by following the Noble Eightfold Path.
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May all beings be happy!