The cost of carrying a burden alone
Frodo separates from the Fellowship specifically to protect others from the Ring. That choice isolates him. The further he goes, the more the Ring weighs on him and the fewer people he can share that weight with. Sam stays, but even Sam cannot fully understand what Frodo is going through.
Loyalty under pressure
Sam follows Frodo into Mordor knowing he may not come back. Éomer stays loyal to Théoden even when Gríma Wormtongue has poisoned the king against him. Loyalty in this book is not easy or automatic. It gets tested, and the characters who hold to it pay a price.
Corruption and resistance
The Ring corrupts differently depending on the person. Boromir broke. Faramir holds. Gollum is what happens after decades of possession. The book keeps asking what makes some people more resistant than others, and it does not give a clean answer.
War as a collective effort
No single hero wins the battles in this book. Helm's Deep is saved by Gandalf, Éomer, and the Huorns together. Isengard falls to the Ents, not to any army. The book distributes agency across many characters and shows that large-scale victory requires contributions from unexpected places.
Hope against despair
Théoden is trapped in despair before Gandalf frees him. Frodo and Sam keep moving toward Mordor even when the odds are impossible. The book treats despair as a kind of trap, something that can be broken from outside or resisted from within, but always something that has to be actively fought.