Well, in the game, it'll get into that more.
But I'll try to put it in a nutshell, because this part is slightly different in the game. Here's how:
You'll bond with Eevee over the adventure, and at a certain point, you'll find out that Eevee is the Pokemon that made you dislike bla bla bla. Sorry, I'm having computer problems ATM so I can't type much without my computer freezing up on me.