Isn't that enough?Well, yes, it can be. During my PhD studies I met a couple people who could be considered "lifers" who were clearly very happy doing graduate-level research work. Maybe you are one of them. Their optimal career path would be a cushy postdoc job so they continue to keep doing what they are doing but without paying tuition fees.
I'm sure you know, but for others reading, there are reasons why most people don't do this.
1. There is a strong psychological toll on doing original research. It's liberating and exciting, but also intensely frustrating and isolating. Writing papers, and their inevitable rejection, is a very disheartening venture that can't be accurately described without doing it.
2. If you're in the sciences, which I assume most people reading here are, you stand to make significantly more in industry research than post-doc research.
3. Research takes over your mind, you can't turn it off. When I was doing my PhD, I was essentially "on" all the time, it can never be a 9 to 5 job. Despite your best efforts, research will always have a toll on your family.
So if these things aren't a problem for you, then it certainly is enough. Research is a uniquely rewarding career path. But your mentor uses that line as a way of helping/warning others. You aren't the stereotypical PhD student, and PhD students are already skewed all the way down the spectrum of "normal". The majority of the population, and most PhD researchers, struggle with these problems. If you're not committed to it for the long-haul, which is what your mentor is alluding to, it's not worth doing it.
EDIT: Now I've done my PhD, people often ask me "Do you think I should do one?" and the unsatisfyingly true answer is "I don't know, I'm not you" (and, perhaps even worse, is that you aren't you when you finish the PhD from when you started, you change and grow a lot, which affects your viewpoint dramatically). That's another reason why your mentor has that saying, it's more easily digestible by those who are unsure.