Unlike previous Mainstream Pokemon Games, a Pokemon's Happiness now plays a role in their Stats. The happier a Pokemon is with their trainer, the more its stats will be boosted. A Pokemon with no happiness at all with receive no boost whatsoever to its stats; however, one with Maximum Happiness will receive a 10% Boost to all of its stats. Depending on a Pokemon's level of happiness, it will receive between a 1%-10% boost to its stats. To know exactly where your Pokemon's Happiness is currently at, you can visit your Rival's Sister in Pallet Town to check.