Have you been playing with your Pokémon in Pokémon Amie a lot? Once you reach higher levels of afffection your Pokémon will start to behave differently in battle. Every now and then it will dodge attacks, recover from status, and get random crits. Aesthetically, you can occasionally pet your Pokémon after a KO and hearts will appear over their heads.
Too lazy to edit this into my paragraph, but:
>Also, it's not just occasionally, you will always get to pet your Pokémon's head after a KO and those hearts do not increase affection. You can mention that hearts appear after every special effect (surviving a KO, shaking off status, landing a crit etc.)
commented 9 minutes ago by DraconicUmbreon02