"Okay, I've got to ask," Brock said. "Ash, how come you still look the same as when we met?"
"Huh?" Ash replied. "Do you think I shouldn't?"
He looked at Pikachu. "I thought I'd changed my clothes a few times, at least…"
Pikachu shrugged. "Ka-pikaaa-chu. Pika-Chu?"
"Oh, yeah, maybe," Ash agreed. "Is that what you mean?"
Brock looked completely lost.
"Ash, I can't understand Pikachu," he said. "Actually I'm not sure how you understand him so well."
"I guess it's just… we've spent so long together?" Ash asked. "It must be, what, ten years now?"
He laughed. "It's weird how much time passes when you aren't paying attention. It just comes one day at a time!"
"Pika-ka-kachuu, Pikapi," Pikachu said.
"I guess so, Pikachu," Ash agreed, reaching down to grab his Electric-type, and transfer him to his shoulder. "Anyway, uh, you wanted to talk about something?"
"Yes!" Brock stressed. "I was originally here to ask what you're going to do now. You've got almost your whole team from all over back together, haven't you?"
He shrugged. "And, even if you weren't going to be aiming for anything in particular after the World Coronation Series, I wanted to know whether you were going to try Contests, or if you wanted help deciding… but now I'm completely sidetracked by this question!"
"I was thinking about that," Ash admitted. "Now I've got Pidgeot back, I can go anywhere really easily without having to rely on Dragonite… in fact, I can swap them around! So I might try finding some cool places with cool Pokémon, or just see where the road takes me."
"Stop getting sidetracked!" Brock complained.
"Huh?" Ash said, scratching his head. "I guess if I knew where I was going then I could be sidetracked from it, but without that…"
"Pikaaaa," Pikachu giggled.
"Oh, yeah, that," Ash said. "It's actually… well, I guess it's not embarrassing, it's just a thing which I kind of thought of as weird for so long that it still feels weird. You know?"
"No, but keep going, or this will take way too long," Brock advised.
Ash nodded. "It's… you know when I got into Celadon Gym?"
"Oh, yeah!" Brock remembered. "That was kind of funny. You looked cute as a girl! But I never figured out why I couldn't tell it was you."
"And you remember Pokémon Tech?" Ash went on. "Because… there was a girl there, and, I tried to be interested in her, like… you know, the way you are with girls. And it didn't really work?"
He shrugged. "And when there's Jessie and James turning up all the time, I guess I started wondering about the difference."
Brock looked blank.
"...you did get the talk, right?" he asked. "Because if you've managed to miss that until-"
"No, yeah, I did get that!" Ash interrupted, waving his hands and blushing. "It's not that!"
"Chu-chuuu," Pikachu giggled. "Pika-pika, pikapi."
Brock wiped his brow. "Phew," he said. "I had to do it for all my brothers and sisters who needed it so far… my parents said I should do it."
"Was that a good idea?" Ash asked.
"Hey, I have the right attitude with the ladies!" Brock defended himself. "But we're supposed to be talking about you."
"Right, yeah," Ash said. "And I guess…"
He sat down, and Pikachu jumped into his lap.
"I guess I just feel like I don't feel the difference between being a boy and being a girl?" he asked. "At least, it's… there, but it's not this big thing. It's not a big thing for Pokémon, most of the time. And sometimes I don't really feel like either, but sometimes I feel like one of them a bit more. Or the other."
He shrugged. "And the way I am, it… works? I don't have to put in all the effort Jessie or James do when they want to look different, because I'm just… me. Ash Ketchum. But if I grew up too much… I might stop being all of me, because being a man would be really different."
"Kaaa," Pikachu said, nuzzling into Ash's side.
"Yeah," Ash agreed, patting his closest friend. "Maybe that's not how it works, but I talked to Professor Oak about it, because he's really smart, and then there was that time Team Rocket took me prisoner and we talked about it for an hour while Pikachu nibbled through the ropes… anyway, I just take these pills. It's part of breakfast now, I don't even think about it much."
He shrugged. "So… I guess that's it? Nobody's ever really asked about it in the last eight years."
"I guess everybody else either already knew or guessed, or never noticed," Brock said, before groaning. "And now I think about it, I really should have noticed when I met you in the last few years."
"Maybe this is the first time you've met me without a pretty girl nearby to focus on?" Ash asked.
"Hey!" Brock protested, then groaned as Pikachu giggled. "But… Ash, should I call you something else?"
"Huh?" Ash asked, then shook his head. "No, I know about that stuff, but I'm Ash, and it's okay to say he. That's something I'm comfortable with, and it'd be…"
He shrugged. "It'd be kind of silly to not use he if it made me less comfortable, right?"
AN:
Well, it's one explanation.
Ash isn't going into all the details, like who the actual doctor was. Among other reasons, it's not strictly relevant.