KKBeckett asked a good question

and, I don't remember if I explained it

So, why did Sora fall into darkness? He was so bright as a kid... what changed?

the very very simple reason?

Riku showing up in this universe threw the universe out of balance. In a desperate need to fix the balance, the universe did practically everything.

Sora falling into darkness was one of those things

There is, however, a deeper level to this (what!? I'm going DEEP on you? NO WAY!!)

and to understand that, we have to go back to when Sora was a kid.

Sora LOVED fairytales. LOVED THEM. His mom would read him a book (or three) every night before he went to bed. His father would take him (and often Kairi, too) out to the smaller island every day so he could play. he'd be a pirate one day (taking Kairi as his captive) a knight the next (protecting "princess" Kairi)

he wanted more than anything to actually be part of that magical world

he wanted to actually fight monsters, be a hero

see where I'm going?

Then Riku showed up.

Riku showed up, and he could fight. He could kill heartless, or "monsters" without breaking a sweat. He used "magic". Of course, it wasn't REALLY magic, but to Sora's unknowing eyes, a Dark Aura looks like a pretty epic magic attack (and, I suppose, basically is. just depends on what you ACTUALLY consider to be magic)

Sora.... Sora hated him for it.

"The magic, the monsters, all of that- it's REAL. And I'm not a part of it."

He saw that it was real.

He wanted to be a part of it to.

He'd stop at nothing to BECOME a part of it.

When you grow up on fairytales and magic, you don't just sit there and let someone who's as good as one of those heroes go. you see them, and you think "what? It's REAL? Why aren't I a part of it?"