"When you recognize that pain — and response to pain — is a universal thing, it helps explain so many things about others, just as it explains so much about yourself. It teaches you forbearance. It teaches you a moderation in your responses to other people’s behavior. It teaches you a sort of understanding. It essentially tells you what everybody needs. You know what everybody needs? You want to put it in a single word?
Everybody needs to be understood.
And out of that comes every form of love."
This seems to imply two broad sources of love: that which arises when we feel understood, and that which arises in the act of understanding.
When we feel that someone understands us, we feel some form of love for that person. This is the appreciation of a child for a parent who is able to help them navigate the world; the gratitude of someone suffering for a person who empathises with their pain; the romantic attraction of one adult to another who finds them in some way admirable. This is love as an emotion.
Choosing to make the effort to understand someone else also gives rise to love. This is the reassurance a parent gives to their child; the support offered to a friend; the admiration of one adult for the another. This is love as a verb.
Conversely, the greatest threat to love, in Nuland’s view, would presumably be a lack of understanding.