Psychologists have scientifically proven that one of the greatest contributing factors to overall happiness in your life is how much gratitude you show.
In this experiment put on by Soul Pancake, subjects who wrote a letter of gratitude, thinking of someone that influenced them the most, saw a rise in happiness of 2-4%.
However, for subjects who picked up the phone and personally expressed their gratitude to the person that influenced them the most in their life, there were happiness increases of 4-19%, showing that expressing your gratitude will make you a happier person.
This happiness experiment in gratitude is based on the following study:
Martin E. P. Seligman, Tracy A. Steen & Christopher Peterson, “Positive Psychology Progress: Empirical Validation of Interventions.”