I think it all comes down to trust. We are so aware of our vulnerabilities and yet so aware of the destructive capacity of humankind in general, and we have often been hurt or disappointed in the past, that we are reticent when it comes to letting anyone into our inner world. We do not want to get hurt, because when we hurt, we hurt deeply. And we are very private people because we know that divulging our hearts puts us at risk for hurt or betrayal. For us, closeness always involves that vulnerability - we do not hold back our inner selves from those whom we consider as close friends (which may be a small subset of the people who consider us their friends) and we desire to have very intimate friendships. So the combination of the vulnerability and the innate distrust of people makes us very guarded about who can get close to us.
Think of it as self-preservation. It's nothing personal, really ~ whether we let you in or not, we still care about you and would help you out if you were in need. We don't like to cling to negative feelings against others and we aspire to be benevolent to everyone. We just keep a guard stationed at all times around our hearts.
What we must see in a person in order for them to break the trust barrier (to be welcomed into our inner lives) that they are worthy of our trust - therefore, complete honesty, authenticity, and a (proven) genuinely kind heart. That's not all, but the rest of the requisite traits would vary by the individual INFJ.
Think of it as self-preservation. It's nothing personal, really ~ whether we let you in or not, we still care about you and would help you out if you were in need. We don't like to cling to negative feelings against others and we aspire to be benevolent to everyone. We just keep a guard stationed at all times around our hearts.
What we must see in a person in order for them to break the trust barrier (to be welcomed into our inner lives) that they are worthy of our trust - therefore, complete honesty, authenticity, and a (proven) genuinely kind heart. That's not all, but the rest of the requisite traits would vary by the individual INFJ.