Let's face facts. From the beginning, it seems like a great deal of the characters in The Return of the King are doomed to death. How in the world can a motley band of Middle-earthers defeat the ultimate Big Bad? It would be understandable if these guys crawled under a rock. But aside from Denethor (whose cowardly despair is his biggest mistake), the inhabitants of Middle-earth rally rather than run away. Even though they believe failure is certain, Pippin, Frodo, Sam, and Merry refuse to despair. They all keep going with their duties, even if they are certain that they will not be successful. It's not that they're happy to; it's that they must. And that's what we call perseverance, folks.
In The Return of the King, characters have to persevere, not for their own survival, but to guarantee the survival of others.
While love helps characters like Frodo, Merry, and Pippin persevere through dark times, the most essential trait for encouraging perseverance is a strong personal sense of duty.