There are some things that people do so often, or so automatically, that it is almost expected. Maybe it is cooking dinner, or cleaning, or a certain task at work, or walking the dog. I have heard from clients who basically say something to the effect of "I did all this work, what they did was the least they could do." While this is perhaps accurate, it doesn’t mean that you shouldn’t strongly consider acknowledging them anyway.
People like to hear acknowledgement. They like to know that what they are doing, and being, is appreciated by others. Regardless of whether the tasks they are doing are expected of them, or part of their job description, they will still appreciate hearing how much it is appreciated.
Make acknowledgement a habit. It makes others feel great, and it’s a great part of being alive.