Happiness has been a key part of the Pokemon games since Pokemon Gold and Silver. Not only for the players, but for the trainer's Pokemon. There are several moves and Pokemon that are effected by this often forgotten game mechanic. Most Pokemon starts with a base happiness of 70. Maximum happiness is achieved at a level above 210 base happiness. First, lets look at the different ways to influence happiness.
Battling
One of the simplest ways of gaining happiness. Pokemon like to be used in battle. In actually, Pokemon only gain happiness by gaining levels, but do not gain happiness from Rare Candies. If you are trying to gain happiness of a baby Pokemon or a weak Pokemon (perhaps from an egg), it is recommended that you use the Exp. Share since if your Pokemon faints it will lose happiness which would be counter productive.
Poffins
Poffins, normally used to increase the visual aspects of your Pokemon for Super Contests, also have a second function. They increase your Pokemon's happiness. The amount of happiness points that increase (or decrease), varies on the individual tastes of your Pokemon. To find out your Pokemon's taste preferences simply locate its Nature on our Nature Guide. Obviously enough, feeding your Pokemon with poffins it prefers will increases its happiness and feeding your Pokemon will poffins it detests will have the reverse effect.
Grooming
Like in previous Pokemon games, there are two different places where you may take your Pokemon to be groomed to increase their happiness rather significantly. The first can be found in Veilstone City, the building directly below the Gym. The lady inside will groom ONE Pokemon per day, along with adding an accessory for Super Contests for your inventory. The other grooming location is in the elite "Ribbon Society", located near the Battle Tower in the northeast corner of Sinnoh. To obtain entrance you must have obtained 10 Ribbons on any one of your Pokemon.
Berries
There are six berries that are also used to increase happiness. These are the same six berries that are used to reduce Effort Values are allocated to your Pokemon. (To learn more about Evs, check the EV Guide.) Kelpsy Berry, Qualot Berry, Hondew Berry, Greap Berry, Tamato Berry, and Pomeg Berry are the names of the "anti-vitamin" berries. (To find the locations and effects of these berries, check the Berry Guide).
Vitamins
Well if Anti-Vitamins increase happiness shouldn't Vitamins decrease happiness? Well your wrong. Just like the berries listed above, Protein, Iron, Calcium, Zinc, Carbos, and HP UP, all increase your Pokemon's happiness. If you plan on using this Pokemon for competitive purposes its good to do this anyway to get some Evs quickly.
Items
There are also two items in the game that can help increase the happiness of your Pokemon. The capturing device, the Luxury Ball, increases the happiness of a Pokemon on capture. In addition, equipping your Pokemon with the Soothe Bell will increase its happiness on every step. These two combined can score big happiness points with any Pokemon.
Checking
Now how do you check to see your Pokemon's happiness without a complex formula to use? Luckily, Nintendo supplied us with certain NPCs in each game to find out. In FireRed/LeafGreen, Daisy in Pallet Town will tell you how much the first Pokemon in party loves you (and give it a haircut occasionally) as well as the lady in one of the houses in Verdanturf Town in Ruby/Sapphire/Emerald. Finally, in Diamond/Pearl/Platnium, one of the Poketch applications will allow you to check the Pokemon's happiness by equating it to a certain size and numbe of hearts.
~Hiddenzbu