Ash is comatose. [ANIME TIE-IN THEORY]

Okay, so Ash goes to Dr. Oak's lab to get a pokemon and they're all taken so he gets Pikachu, but Pikachu hits him with a lightning bolt.

WELL HE GOES INTO A COMA BECAUSE OF THE LIGHTNING BOLT.

That's why he's 10 years old forever, and why Pikachu likes him all of a sudden.