Why does my Greninja get boosted EXP?

While evolving a Sylveon for a trade, I stumbled upon a solution. Once I was ready to level up my Eevee, I noticed that he too was receiving boosted EXP. As it turns out, I had messed around Pokémon-Amie with Greninja and he ended up with 2 hearts of enjoyment. To verify this hypothesis, I played with a spare Gabite, defeating a wild pokémon after each additional heart.

With a single heart, there was no noticeable change to the Gabite: she still earned normal EXP and didn't seem to have the thelepathic abilities Sylveon displayed. Once she earned her second heart, she immediately started earning boosted EXP. She also starting dancing at the beginning of battle as Greninja had.

Earning two hearts of Affection in Pokémon-Amie will boost EXP earned for the beloved pokémon

Boosted EXP without being traded may be a result of the EXP Point O-Power, which somebody might have buffed you with. If you were connected to the Internet through the Player Search System (PSS), this is the most likely case. To get this O-Power yourself, you must talk to Mr. Bonding in the Anistar Pokemon Center.

Also, it seems that increasing your Pokemon's affection and related stats through Pokemon-Amie has a chance to increase EXP gained along with a host of other effects.