I was at a lecture by Richard Dawkins a while back, and he specifically brought up the topic of gratitude as a human parallel for (I can't remember the exact name he gave it, but wikipedia calls it Vacuum Activity). Just like a dog trying to bury a bone in the corner of the room. It also had to do with our inherent patter-matching nature, and the survival difference between making Type 1 and Type 2 mistakes (where if you falsely believe a tiger is about to pounce you, you pay a small energy cost from freaking out and running away; but if you falsely think it's just the wind, you die).