I had an internship during the summer at a small software company. I LOVED the environment there -- the people I worked with were fantastic, smart, and really enjoyed what they were doing.
At the same time, I was working on an undergraduate thesis research project with a faculty advisor at the university. It was difficult and required an extra 2 hours per day of work on top of my full-time internship.
The big "aha!" moment for me was when I'd work a full day at the company, and then find that the highlight of my day was my afternoon advising meeting where I discussed research ideas and progress. I enjoyed the industry work, but I didn't see myself enjoying it as much as the people around me did. I am using my skills most effectively when doing work in which I am personally invested, and working in industry did not allow me to be personally invested in my work.
Finally, I read the following professor "job description" (from the CMU "grad school talk" document):
"If you choose to be a professor at a research university, your life will consist of the following tasks: (i) doing research on anything you like, (ii) working with graduate students, (iii) teaching classes, (iv) applying for grants, (v) flying around to work with other researchers and to give talks on your research, (vi) doing service for your department and school (like giving this talk). Note that I say “your life” rather than your job, because for new faculty, your life becomes your job. It’s a fantastic job/life for me because I love these activities, so I’m happy to work hard at all of them, but it’s not right for everyone. "
This sounded absolutely perfect to me. I could imagine going without a PhD, but I would never have the chance to work as a professor and have a job/life like this.