I think this is because my job and virtually all others work one area of the brain more than others. Those other parts of your brain need to be exercised, even if they're not necessarily your strong suits. Similar to going to the gym after a spell of physical stagnancy or listening to your favorite local radio station after being on vacation, you feel more whole when you utilize all pieces of yourself.
Another advantage of taking a break from your normally all-encompassing world (work or maybe even a relationship or romance in general) is the chance to gain perspective on it from the outside. When talking to relatives, dates, or those in my parish, it never ceases to amaze me how cool they think my job is. None of them focus on the logistics of organizing 900+ samples in triplicate or the challenges of making sure everyone on the project is up to date on what's going on. These outsiders see that I'm working with unique strains of bacteria awesomely adapted to deal with extreme dryness and salt. Who's to say which of these my job is really about- the impressive big picture or the ordinary nitty gritty? I'm the one who gets to choose how I see it, and that makes all the difference.
PS his name is Harry.