Ask HN: 'Do what you want'... Does it really matter what you want?

This is perhaps the most stated advice, the fall back, in many instances, of giving some advice, the first thing that comes in mind when one asks as to what they should do. Do what you want most, what you have a passion for, absent of any consideration that most people, if not all people know not what their passion is. My question is the simplest, and I hope you will engage in debating it. Does it matter at all what you decide to follow in the absence of any clear knowledge of what you want or what your passion is? Is there such a thing as passion and want when deciding how to spend your life and choosing your career, or is instead our path paved by circumstances and events, our passions formed by our experience rather than carved by our own controlled paving of the future?