To me at least, it seems to tie in with the fact that, due to our particular blend of nature-nurture, many of us INFPs consider ourselves to be meaning-driven individuals (even if that means creating our own meaning), constantly calibrating and reflecting upon what really matters to us. For many of us there are very specific factors that need to align in order for us to be motivated, so we are constantly seeking purpose in things and subsequently (whether consciously or subconsciously) creating a framework of things that truly matter to us (which might be these so-called "values"). Things that truly matter to us could include open-mindedness, authenticity, kindness, respect for human dignity, logic, love, all kinds of things. They're just things that align so strongly with the core of our being that we often feel the need to defend them if threatened.

I think most INFPs are open to their values changing/being modified over time if need be; it's the more universal/basic ones (open-mindedness, authenticity, etc.) that seem to be pretty steadfast.

For me personally, the foundation of what matters to me, the thing that permeates throughout all of my "values," is a sense of 'live and let live' and not arbitrarily asserting power or control over others. This manifests in many ways, but it's at the heart of what I find meaningful in life.