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Ultimately, the reason why love and compassion bring the greatest happiness is simply that our nature cherishes them above all else. The need for love lies at the very foundation of human existence.

Dalai Lama XIV, Rajiv Mehrotra (2010). “In My Own Words: An Introduction to My Teachings and Philosophy”, p.3, ReadHowYouWant.com
Ultimately, the reason why love and compassion bring the greatest happiness is simply that our nature cherishes them above all else. The need for love lies at the very foundation of human existence.