It is not everyone who has your passion for dead leaves.
We can guess that the context has someone who likes to look at or keep dead leaves (maybe for the fine artful network of veins left behind after the rest decays away), who also tries to get someone else interested in dead leaves, prompting this sentence in response. It parses as follows:
It is { not everyone } { who has ... }.
It is X [subordinate clause].
This is sometimes identified as a type of cleft sentence, where "It is" is a fixed construction that does not change even if X is plural:
It is they who are lying.
Other examples with different X and different kinds of subordinate clauses are:
It is { she } { to whom we owe our thanks }.
It is { he } { whose bag has gone missing }.
It is { for this } { that I have come }.
It was { those books } { that you gave to me }.
And here are some examples in question form:
Is it { she } { to whom the books belong }?
Who is { it } { whose bag has gone missing }?
Is it { for nothing } { that I have come }?
Which books was { it } { that you gave to me }?
A more complicated example:
It is { not everyone who speaks } { who understands what they are talking about }.
Anyway it is indeed not easy to analyze this kind of construction, and even native speakers might not be able to give an explanation. Grammarians still disagree on what exactly the subordinate clause is. All that you need to know is how to use them, which is that the subordinate clause follows the same rules as relative clauses.
I used this phrase on myself when my marriage ended for reasons that I knew existed prior to but chose to ignore or had a notion I could change. In the end, I said, " Well, I went into it with my eyes wide shut".
Therefore, the definition to me is very much a play on words. It doesn't only apply to a naive individual. It applies to those who choose to enter into or take on something and all the while are fully aware of the perils or dubious outcomes that may lie right before them. It's a connotation that could be one of romanticism or that of ignorance to the obvious and not listening to intuition.
Best Answer
It is just a cryptic/clever/verbose way of saying that "who are you" just means "what are you" ("who" is just another word for / has the same function as "what"). To which he responds that he is just a man in mask.
Form follows function is a term from 20th century modernist architecture, and has influenced many aspects of modern design.