Good trainers always want to keep their Pokémon content, for angry Pokémon do not want to help you on your journey. Strangely enough, there actually is a happiness value in Pokémon games that keep track of how happy your Pokémon are! Happiness affects your Pokémon in different ways, including the power of certain moves and their evolution. 
 
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Introduction

In the Pokémon games, happiness is a value that can be measured on a scale of 0 to 255, with 255 being the happiest a Pokémon can be. Pokémon can gain or lose happiness, based on what you do to them. Basically anything you do that seems right or nice will increase your Pokémon's happiness, and anything mean or malicious will reduce its happiness. For more details, visit the Affecting Happiness section. Most Pokémon start with a base happiness of 70, with hatched Pokémon starting off with a slightly higher base, and legendary Pokémon starting off at a lower base. Trading a Pokémon will reset the Pokémon's happiness, so be aware when you trade!
 

 
First off, to find out how much your Pokémon loves you (its happiness value), you have to visit a certain house. In RSE, visit a girl in Verdanturf City and in FRLG, visit Daisy (your rival's sister) in Pallet Town. In DPPt visit the Fan Club in Hearthome City and talk to the lady inside. You can also see Dr. Footstep of Route 213 (found in his house by the sea), who will give your Pokémon a ribbon if it has maximum happiness. Also, Pokétch App #6 allows you to touch your Pokémon and see how many hearts it has; the more and bigger the hearts, the better. In HGSS visit a girl in Goldenrod City.

 
To raise your Pokémon's happiness, just do one or more of the following. Remember that raising happiness will increase the power of the move Return. Click here for more information about the move Return.
 

	Catch the Pokémon in a Luxury Ball.
	Hatch the Pokémon from an egg instead of catching it from the wild.
	Attach a Soothe Bell to the Pokémon and walk a very long distance. The farther you walk, the higher the happiness goes.
	Use the Pokémon in a battle, win, and level up. Leveling up a Pokémon will increase its happiness greatly. Basically, walking around and carrying your Pokémon will raise its happiness.
	Feed your Pokémon with items that raise its happiness. These items include Fresh Water, Lemonade, Soda Pop, Moo Moo Milk, Poffins (which you can make in Hearthome City), Vitamins (Protein, Calcium, Iron, Carbos, HP Up, Zinc), and EV reducing berries (Kelpsy, Qualot, Hondew, Grepa, Tamato, Pomeg).
	Win a Contest or Super Contest with your Pokémon.
	Get a massage from a masseuse. In FRLG she can be found in Pallet Town, in DPPt the massage center is found in Veilstone City or the Ribbon Syndicate in Resort Area, and in HGSS he can be found in the Goldenrod Underground.
	Walk with your Pokémon at Amity Square (in DPPt only). The longer you walk, the higher the happiness level goes! However, remember that only "cute" Pokémon are allowed to walk here.

 
In Pokémon DPPt you can use the Pokétch App #6 to check how much your Pokémon like you! Simply tap the Pokémon, and depending on the number of hearts (with 2 big ones being the best), you can see how much your Pokémon adores you!

There are many Pokémon that level up depending on their amount of happiness. Some even have more conditions to level up. Please note that happiness only applies to the GSC generation onwards.

Not only does happiness help Pokémon evolve, but it also affects the power of two moves, Return and Frustration. Depending on how high the happiness of a Pokémon is, its Return or Frustration power will vary. Maxing out your Pokémon's happiness will render Return a very powerful move, with an attack base of 104. If your Pokémon has no happiness at all, Frustration will have a max power of 104. Obviously, it is much easier to max out Return, as raising happiness is easier than losing happiness.