I hate to sound cynical, but it's impossible to befriend every single one of your Pokémon, just like it's impossible for them to all become friends with each other.

Sometimes, you'll end up with a Pokémon that has a nasty attitude outside of battles. That's important to keep in mind because it means you'll have to work around that.

[He shakes his head.]

Take it from someone with three years of experience; Pokémon are as diverse as people, and some are just as cruel.