Let's face it, we all have "those days" at work. No matter how much you love what you do, there are just some days, that to put it bluntly, bite.
Today wasn't extraordinary in any way until the end. A client came to my office. He was initially a bit hesitant, but left with a genuine smile on his face and a definite look of appreciation. His happiness made me appreciate him. That is what I would term job satisfaction.