Wow, the royal family in Rohan is really having a bad time in this one. We spent most of Éowyn's "Character Analysis" talking about what a rough life she has, and now here we are, talking about the miserable existence of her uncle, the king of Rohan. The thing is, by the time we meet King Théoden in The Two Towers, he has been possessed for a long time by the powers of Saruman. Saruman has used his nasty little advisor, Gríma Wormtongue, to get close to Théoden and to influence his thinking. As far as we can tell, these spells deaden Théoden's feelings. He has a tough time truly understanding what's happening around him.
While Wormtongue is around to numb Théoden's mind and heart, a boatload of bad goes down in the kingdom of Rohan. The orcs of Isengard keep making raids, so Théoden's people are suffering. Éomer, his only nephew, gets thrown out of Rohan by Wormtongue. And worst of all, Théodred, Théoden's only child, gets killed in an orc ambush. All of this terrible stuff has happened before we even meet Théoden in the book. So when Gandalf finally gets rid of Wormtongue and clears Théoden's mind, Théoden regains consciousness only to find that his world is falling apart.
Théoden may be cured by Gandalf breaking Wormtongue's spells, but he isn't 100% healed. Not only has he spent months and months struggling with Saruman's influence and Wormtongue's despair, but also his only son died while Théoden was in his funk. Théoden's a human being, so even though he continues to lead his people with a lot of courage, he also can't just bounce back from all of this bad stuff that happened to him. He has to readjust to the newer, darker world.
It's a good thing, then, that Théoden is a brave guy. Plus, he believes in honor and in the protection he owes to his people as their king. Once Wormtongue is out of the picture and Théoden has returned to himself, he is totally gung-ho about helping to destroy Isengard's orcs and then riding on to protect Gondor from Sauron. But he's not as solid as his nephew, Éomer. He still has brief periods of doubt and uncertainty in the middle of battle.
Those months under Saruman's power have left a mark on Théoden's character. And to be honest, Théoden's moments of self-doubt and depression are actually among the reasons we really like this old guy. Maybe he isn't as tough a leader as Aragorn, but he seems a lot more human and approachable than, say, Denethor, the cold Steward of Gondor.
One last word on Théoden: he treats other people really well (when he's not being possessed by Wormtongue, that is). When Théoden first meets Merry in the ruins of Isengard, he greets Merry politely and listens to him carefully, even though Merry's a mere hobbit. Now, think about it, Théoden is the leader of an army and a whole country.
We're sure he's got plenty to do besides listening to hobbits talking endlessly at the foot of Orthanc. But Théoden respectfully pays attention to Merry chattering on about different kinds of tobacco like he has nothing else on his mind. Théoden's politeness and courtesy might be a bit old-fashioned, but they impress Merry deeply. And that's the beginning of a beautiful friendship, which we'll see more of in the next book.