How does the author define "unrequited love" as noted in the nonfiction book, After the Affair?

The author defines unrequited love as a powerful but irrational attachment to a partner. Unrequited love makes an individual desire to remain in a relationship even when the relationship is clearly dysfunctional. The text suggests that unrequited love is a major reason why many hurt partners remain with unfaithful partners even when the unfaithful partner's behavior is repetitively abusive and unlikely to change.