There's no brainwashing involved. He's just being honest with himself now. You know it's true. I mean, for god's sake, he likes Hyatt. You ever wonder WHY he has such a morbid fascination with her?
Watanabe was a dick in Episode 26 of the anime, too. Think about it.
That's not true...
The following is a spoiler regarding stuff form Volume 15:
It's clearly seen (actually, also read) in Volume 15 of the Manga, that Kabapu has done something with Watanabe for him to get in this mood...
When Misaki asked Kabapu what he had told Watanabe, he dodged the question and got away... He hasn't really been questioned ever since
Who knows, perhaps it IS some kind of self-realization (is that a word..?)But Kabapu most certainly has SOMETHING to do with it, therfor, very well being involved in Watanabe's current state...
Perhaps it's what sumiyoshi suggested in that volume, that they aren't allowed to be depressed anymore haha
(although, i find that a little too unlikely...)