“Proof that Tony Stark has a heart.”
It was Pepper alone who stood by Tony when the whole world called him crazy. It was Pepper who went crazy worrying that her boss would go and get himself blown up, and it was Pepper who saw the heart, the emotionally charged side of Tony that he covered so, so well.
“You’re all I have too, you know.” In the end, the last person standing faithful was Pepper because she cared so much about the kind-hearted and determined side of Tony that no one else would care to look twice. Slowly, carefully, Pepper won the heart of our briliant little billionaire; the heart she slowly helped him learn he had. And in return he made it into a promise with a small, heart key necklace, a forever.