The easiest way to make any Pokémon happy is to attach the Soothe Bell to it (obtained at the Pokémon fan club in Slateport), and keep it in your party. Then, using the Acro Bike, have the up button held down while attempting to ride up a slope - one of the slopes that you can go up only while on the Mach Bike. Leave it like that overnight, and it'll have max happiness in the morning.