In my mid twenties this was hard for me to grasp. I was working as an electronics design engineer for a company that made low-end industrial controls. We had a customer who had what seemed like the dream company to me: he made robots for animatronics and lab applications. One day I was talking to him about how exciting the work was and his answer surprised me: it used to be fun in the beginning but after doing it for a few years, it was just another job.
I work with wireless sensors now. Yes, it's technically interesting, but it's pretty boring.
Why? I've discovered over the years that I am far more excited by making customers happy than by doing technically interesting work. I've come to see the technology as a means to an end, not the end in itself.