This is a repost of my answer to the Quora question: What are some strong motivations for earning a PhD?

You want to earn a PhD because...

• You want to surround yourself with the best, brightest, and most motivated people on Earth, and in so doing, push yourself to become one of them.
• You want to get paid to (mostly) have free reign to explore your own ideas, to fail without consequences, or to try something that doesn't have to work—as long as it expands human knowledge.
• This video looks like heaven to you: rocks, bands, logic (2012)
• You relish being able to say "I just got home from the lab" instead of "I just got home from the office".
• You want your life to be science fiction.
• You want to live on the very precipice of human knowledge, and witness new discoveries firsthand, years before the general public catches wind of it.
• You've looked everywhere and you just can't find the stimulating intellectual atmosphere like that of a university. Your coworkers just don't seem to think about or care about the same things as you.
• No matter what high-tech goliath or trendy startup or progressive non-profit you work for, it's just not fulfilling. It appears to you that fulfilling industry jobs do exist, but they almost all require a PhD and involve at least part-time research.
• You constantly challenge and push yourself. You relish being around people who do the same.
• You want to continue growing and learning at a breakneck pace. (It is possible to do this in the workplace, but far from automatic. Being in school forces you to grow due to the incredibly steep learning curve. See here about the importance of maintaining a steep learning curve: Edmond Lau's answer to Career Advice: How do you know when it's time to leave your current company and move on?)
• You tried briefly to follow research on your own, but it's just too hard to navigate. You need someone to guide you into the field. You need a set of strong mentors to teach you how to read a paper and how to identify the important results in the field (be it science, history, philosophy... even literature).
• You want to work at pretty much the only place outside the army that will demand everything you can give and force you to "Be all you can be".
• You want to own all of your work. You decide what you're going to work on and whether to give it away for free or start a company out of it. Unlike the army, you're not a cog in someone else's machine here.
• You want a job that will have you (1) travel all over the world and (2) meet people who are the best in the world at what they do.
• You want to be part of a global community (science transcends borders; same for other scholarly disciplines).
• You want a job that will train you in (or force you to learn) all kinds of invaluable life skills: technical writing and communication, delivering presentations and speeches, teaching and mentoring, etc. (It's pretty hard to get such a wide range of leadership skills in any other entry-level job!)
• You want your contribution to the world to be in the form of knowledge. In their careers, most people render a product or service to the world. Your job is to render knowledge, and publications are the medium.
• And of course, you want a job where you can answer questions on Quora instead of working for a day. ;-)

This might be a slightly romanticized version, but you asked for some strong reasons, and these mostly hold true. They may not all apply to you, but the biggest advantage of a PhD is the flexibility. It really is what you make it out to be; you can become more involved and practice more leadership skills, or you can put your head down and bury yourself in your research. You choose how often you skip out on work and go to the beach. You decide who to collaborate with, and when to go on vacation.

Whichever combination of these reasons appeals to you, they should obey this one overriding rule: You do a PhD for the experience of doing a PhD. You don't do a PhD for the job that comes afterward. Some people go on into very lucrative jobs, some start companies out of their PhD, and some struggle with low wage for the rest of their lives. Whatever comes after the PhD, is whatever comes after the PhD. You do the degree for the degree itself.

People often ask me what I want out of my PhD, what my end goal is, why am I doing it, what job title am I looking for? Well, of course I have some ideas about that—too many ideas, in fact—but for now, for right now, the answer to their question is actually: I want the job title of PhD Candidate. Seriously. There is nothing better I can imagine doing with my life right now than being paid to dig deep, being paid to learn, to take classes, to take on outlandish projects, to soak up knowledge like a sponge. I have to live on a very meagre salary and work very hard, but in return I get this incredible job with flexible hours and amazing colleagues from all over the world. If I'm lucky, I'll get to continue doing that after I'm done, and I'll work in a place with the same invigorating environment and the same clever, diverse, interesting colleagues. I can only hope.

Disclaimer: I'm currently only in the first year of my Masters, but already grad school has been the most rewarding and transforming experience of my entire life, and I don't expect it to change anytime soon.