OK, so I think the difference is ultimately where personal identity is assumed to emerge from:
Fe-types understand identity in terms of one's position in relation to others. From this standpoint, your identity is in jeopardy unless you accept others into your life - you are defined by the community that you choose to represent and be a part of. Until then, you're just an anonymous John or Jane Doe, that might be able to think, move and talk autonomously enough, but still not have a true sense of self.
Fi-types understand identity as an internal, innate "guiding light" and they take solace in simply "being themselves". They don't do it to spite others like unhealthy Fe types - they're just trying to stay in touch with who they are as individuals. From this standpoint, everyone is unique - everyone has their own light that they must follow, and to betray it for arbitrary external reasons is to betray who you are.