I've heard it said that graduate school is like a pencil sharpener, with the graduate student as the pencil. It slowly hones your mind, until you know more and more about less and less, and at the end, you know absolutely everything about nothing. You do research for years about one specific topic, narrowing down as much as you can, until your entire world is encompassed by arcane terminology and esoterica. It feels like only 20 people in the world understand your thesis topic, and only eight of them care. After all that, it's difficult to keep your focus on why you entered the Ph.D. program in the first place, and especially to retain your enthusiasm.
All grad students suffer from that ennui at points in their career.
This focus on research is only to be expected. The main quality academic search committees look for is demonstrated expertise in research, so doctoral programs require students to perform lots of research, while providing a heavy grounding in theory and some experience as teaching assistants. However, there clearly aren't enough professorial jobs to go around, so many graduates are left with somewhat misplaced training when it comes time to seek a job outside of academe. Some engineering professors supplement their income by consulting for businesses, or even spend the majority of their time there, and such contacts and advice have been very valuable to me. Even though most of the fliers in the department were promoting academic positions, I did have some advice and knowledge on applied positions.