Those who are happiest are those who do the most for others. Meaning?

Washington repeatedly expresses the sentiment communicated in this quote. For instance, when describing Hampton, he notes that "at that institution [he] got [his] first taste of what it meant to live a life of unselfishness, [his] first knowledge of the fact that the happiest individuals are those who do the most to make others useful and happy" (28). Later on, he reiterates: "In meeting men, in many places, I have found that the happiest people are those who do the most for others; the most miserable are those who do the least" (87). This repetition suggests that this idea - that serving others leads to happiness - is one of Washington's core beliefs, influencing his own dedication to the Tuskegee Institute.