The more practice you give your brain at feeling and expressing gratitude, the more it adapts to this mind-set — you could even think of your brain as having a sort of gratitude “muscle” that can be exercised and strengthened. If this is right, the more of an effort you make to feel gratitude one day, the more the feeling will come to you spontaneously in the future.

It also potentially helps explain another established finding, that gratitude can spiral: The more thankful we feel, the more likely we are to act pro-socially toward others, causing them to feel grateful and setting up a beautiful virtuous cascade.

— Christian Jarrett