It comes from the Latin word for grateful, and I think it is the most endearing of all human (and some animal) expressions. Some life-changing decisions have been made, either because someone wanted to express it or because someone else showed it.
My sons’ displays of it touched me to the core, and one incident comes to mind, my youngest son’s purchasing of a dental bridge so I could enjoy my meals better. He volunteered to get me dental implants, which are expensive, but didn’t prove feasible because of my jawbone structure. Nevertheless, the will was there, and while we can attribute his action to love, I think it was love born of gratitude.
It is even more precious coming from someone unexpected. A whale recently freed from enveloping fishermen’s nets by a scuba diver with scissors, returned the following day to his boat and floated next to it in gratitude. Gratitude is a mover of humankind especially, though.