[pekk]

People say this kind of thing when they don't have experience of what it's actually like.

A PhD is not a continuation of undergraduate or independent study in a subject. It is a career with the tedium of other careers, and then some because so many other people are trying to do the same thing you are. You may love the subject now, but that doesn't mean you are going to be able to do the research you want. At various points in your career you have to do what your adviser wants, manage departmental politics, get funding and crank out publications. Maybe love of the subject helps you stick to it, but more likely it doesn't make a lot of practical difference once it's a tedious job you do every day.

The pay after spending an equal amount of time in any other career with the same background is likely to be better.

[Brashman]

I wish we would stop referring to doing a PhD as "school" and the people doing it as "students". Call it what it is. They're junior researchers. I get that doing a PhD is also meant to be a training process, but working in industry also involves training and learning.

[analog31]

Kind of like "student" athletes, with presumably the same outcomes?

[Fomite]

This. If you love a subject, go and spend a lot of time at a library. Fondness for a subject is a prerequisite for getting a PhD, but is very, very far from being sufficient.