Sometimes the smallest of things need the biggest of thanks. Once I was told that the reason why we wave to drivers who let us cut in or pass is that if we don't, the person might feel their action was unacknowledged, and may not do something as nice again.
Small things lead to the big things. How can we get anywhere if we don't thank the person who switched washed clothes to the dryer in the middle of the night so we'd have clothes the next day,or when our parents consider a new coat, but deny it for the reason of it not being enough protection against the cold, or when they bring a hot chocolate for you when it is chilly outside?
It's not just about saying thanks for the big things, sometimes its the itty-bitty things that you might not even see. The things that make up the basis of our lives, even if we don't realize it.