Happiness is one of a key part for evolution for certain pokemon. every time your pokemon win a battle, it isn't just gain exp points, but it gain happiness points too. after your pokemon reach it's max happiness point, it will evolve. if you want your pokemon happiness grow faster, here my advice!
-put your pokemon in the first party
-win every battle and don't lose, and
-if you have an item called "soothe bell", give it to your pokemon to hold, it will raise your pokemon happiness faster!
hope this usefull...