Gratitude and kindness appear to feed each other in a loop: receiving genuine kindness tends to prompt gratitude in the recipient, and that gratitude, in turn, tends to make the recipient somewhat more likely to act kindly toward someone else, sometimes an entirely different person unconnected to the original exchange.
This means a single kind act can ripple outward well beyond its original recipient, since the gratitude it produces functions less like a one-time thank you and more like fuel for a chain of further kindness that neither the original giver nor the eventual downstream recipient will necessarily ever witness directly.
Understanding this loop gives a practical reason, beyond simple decency, to practise both gratitude and kindness deliberately, since each one appears to genuinely reinforce the other, producing a combined effect considerably larger than either practised entirely on its own without the other present. This tends to matter most exactly when it feels least urgent to bother with.