Though it wasn't part of his explicit advice, your location is not constant, especially if you've quit your job. Thus, the limiting factor isn't where you are, it's where you could be.
That depends. If you have a family, kids in school, a mortgage, strong family ties, friends and so on that really counts for a lot to a large number of people, and that means that where they are is where they'll stay.
When you're young, unattached and without too many responsibilities this is a lot easier (and possibly even hard to imagine that one day it may not be that easy).
Yes, all of those things affect your ability to move and where you can move to. Whenever you make a major change to your life, some parts will get better and others worse. It's important to recognize what's actually important to you, and act to make the things you actually care about as good as possible.
Given the question, I assume that the thing he most cares about is getting a more enjoyable job,
When you're young, unattached and without too many responsibilities this is a lot easier (and possibly even hard to imagine that one day it may not be that easy).