I've spent a lifetime working on my invulnerability. Generally you can't hurt my feelings with a barge pole. I'd have to care.
And that's the part where I don't fully understand my own criteria when I decide to let someone in, and give them permission. What, out of the millions on Earth, suddenly makes another person special to me so that I care deeply what happens to every teardrop?
In The Gods Must Be Crazy part 2, a wolverine attacks our hero, and stays determined to pursue the attack for so long that a strange bonding occurs that sees the wolverine being carried so as not to die in the outback. I've not had that happen, but it's a recurring theme in literature as long-term adversaries develop respect bordering on affection. Sherlock needed Moriarty. Batman needs Joker.
For me, it more often comes out of my Catcher in The Rye complex, but eventually moves on to role reversal. I set out to help, and in the process am helped myself. It's trading the pain for something that fulfills.
And no one ever gets expelled, even in the face of disappointment. Inside is forever.
The shining moments make it worth it.