"The secret of a long life: Laughter" implies that laughter and a sense of humor contribute to a person's well-being and longevity. It suggests that finding joy and humor in life can promote overall health and longevity.
"The secret of a long-lasting relationship" parallels this idea, suggesting that laughter and shared moments of humor also play a vital role in the endurance of a romantic partnership. It highlights the importance of enjoying and having fun together as a couple.
In essence, this quote underscores the significance of laughter and a positive outlook in both personal well-being and maintaining a strong and lasting relationship. It suggests that finding moments of joy and humor can contribute to a fulfilling and happy life and partnership.