Cheerfulness as a Life Power is a short, hortatory book that makes one practical case: a steady, sunny temper is a real force in a person's life, worth cultivating with the same seriousness as health or skill. Marden opens by treating laughter and good humor almost as medicine, citing doctors who credit cheer with aiding digestion, circulation, and longevity, and arguing that a habit of looking on the bright side keeps the human machine running well.
Against this he sets worry, which he names the great destroyer of his hurrying, competing age. Borrowing from physicians, Emerson, Beecher, and others, he describes anxiety as something that kills as surely as a wound, working by the constant repetition of one disquieting thought. His point is not that life is easy but that the small daily fears and vexations, more than great calamities, are what cloud life and waste a person's energy in friction that grinds out nothing.
Marden then carries cheerfulness into work. He pictures good humor as oil on business machinery: the calm, kindly, even-tempered worker accomplishes more than the one who frets and bustles, and the employer who diffuses good cheer gets better service than the fault-finder. Grumbling, he says, requires no talent and only makes the grumbler more uncomfortable. A habit of cheerfulness is treated as a kind of fortune for anyone just starting out.
The middle chapters turn toward daily life and the self. Marden urges readers to take their fun every day rather than save it for some holiday, and to extract happiness from the actual conditions around them instead of waiting for ideal ones. He retells the story of a stern widow taught by a photographer to brighten her own face, using it to argue that a pleasant look is something worked from the inside, since every emotion shapes the body and the way to be beautiful without is to be beautiful within.
He also notes that happiness tends to arrive sideways. Quoting Humboldt and others, he holds that those fixed on their own happiness miss it, while those who do their duty and serve others find joy comes of itself. The closing portrait is the sunshine-man, the genial soul whose mere presence cheers a whole train and disarms prejudice. Marden ends by recommending air, light, and sunshine for body and spirit, and by holding cheerfulness up as the steady daylight of the soul rather than a passing flash of mirth.