“A man can achieve happiness only by pursuing the happiness of others, because it is only by forgetting about his own happiness that he can become happy.” – D.C. Lau, translation of Lao Tzu’s Tao Te Ching.
Sometimes referred to as ‘the law of reversed effect’, you can easily see the truth in this notion.
To get, you must first give.
To achieve more, you must do less.
To become happy, you must become selfless.