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Letting go gives us freedom, and freedom is the only condition for happiness. If, in our heart, we still cling to anything - anger, anxiety, or possessions - we cannot be free.

Thich Nhat Hanh (2015). “The Heart of the Buddha's Teaching: Transforming Suffering into Peace, Joy, and Liberation”, p.78, Harmony
Letting go gives us freedom, and freedom is the only condition for happiness. If, in our heart, we still cling to anything - anger, anxiety, or possessions - we cannot be free.