Jeeves and Wooster (novel)/WMG

Bertie isn't as nice as he seems.
In reality, he's a twisted version of The Pollyanna. His cheerful demeanor is due to some serious repression, resulting from a lifetime of mistreatment by his friends and relatives. The reason he goes along with whatever his aunts/the Drones/Jeeves say is that, if he examines his own desires too closely, his wall of repression will collapse and he'll end up going on a murderous rampage. In all likelihood, his apparent stupidity is also a front to protect him from the reality that everyone he knows is a Jerk Ass. This explains why someone as "mentally negligible" as Bertie is such a brilliant writer. ""Leave me. I would be alone.""
 * Until he becomes so wearied with the act that he completely sheds his cheerful persona, moves to America, and becomes a brilliant, JerkAss, diagnostician.
 * He'd be at least one hundred by then. House, however, would not be... Maybe if Bertie found some way of traveling in time?
 * I have to say, I half believe this theory. It seems even more plausible in the books, and I find myself reading between the lines for Bertie's hidden angst.
 * My own theory holds that tea works on Bertie like a drug. You know how he acts before he has his tea? That's his real personality. Notice how it's a lot more imperious and self-assertive.

There is no Jeeves.
He's just the Imaginary Friend of a sad, lonely man who lives alone-- a kind of Anti-Tyler Durden, if you will.

The books are a tribute to all the author's dead friends.
The books are in Christie Time- and full of feckless, foolish, harmless, silly, innocent men. Since the great war didn't happen, they happily avoided dying in charges on entrenched positions, choking on mustard gas, and suffering from PTSD. Instead, shenanigans and fancy clothes. Sort of an Alternate Ending for the Edwardian era.

Jeeves is robbing Bertie blind.
What other reason would Jeeves benefit from serving Bertram Wooster? It's obvious he has aptitude for The Plan (pretty much every story is solved thanks to Jeeve's pulling one of this behind Bertie's and the reader's) and he has admitted that has accepted the fact he will have no other job after this one... one has to wonder who manages Bertie's finances...
 * Highly plausible. I might add that Bertie is pretty free with his money and can't say no to anything, so Jeeves probably does this with his master's permission. (In one story, Jeeves owed someone money and asked to borrow fifty dollars. The answer was, of course, yes.)

Bertie has dependent personality disorder.
Let's run through the symptoms. Extreme Doormat, unambitious, zero confidence in his own abilities, openly lets his valet run his life for him. (His reasoning being that, after all, Jeeves is better at it, right?) Afraid of what will happen if he ever loses Jeeves. Constantly seeks sympathy. Stays at the houses of people who can take care of him when Jeeves isn't around. In something of an Establishing Character Moment in "Extricating Young Gussie", he drifts around New York City looking for someone to help him, despite not knowing anybody there, and proceeds to "put [him]self unreservedly into the hands" of some random bartender.

Yes, I got the link to the Wikipedia article from the Living Emotional Crutch page, but it seems to fit like a glove.

Bertie is an Unreliable Narrator.
He's Obfuscating Stupidity; Jeeves, if he exists, is actually just an ordinary, unremarkable valet. Bertie is just reattributing a series of schemes that he thought up himself.

In Thank You, Jeeves, Jeeves was planning to go back to Bertie all along.
He initially gave his notice because he hoped it would make Bertie change his mind about moving to the country. When that didn't work, he signed up with Chuffy, having deduced that Bertie wanted to rent a house from him and knowing that this would guarantee his (Jeeves's) presence exactly where he'd be needed. He knew enough about Bertie's psychology to realize that the banjolele obsession wouldn't last, so he set everything up so that he could keep an eye on Bertie and rejoin his service as soon as the hated instrument was retired.