Maxing out the happiness of a pokemon is somewhat time consuming. Walking around with the pokemon in the lead position will help slowly increase the happiness. You can also feed the pokemon poffins to help raise stats and happiness.
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with raising the Pokemon with that Pokemon for many time You can raise your Pokemon happiness by defeating many trainers with Pokemon do you want to raise.
when you talk to it if it has six hearts around it that means its at max happiness
you can check a pokemons happiness in Diamond/pearl by the use of a poketch app, in previous games you could only know in the happiness was maxed by obtain a ribbon from a certain character depending on the game.
Either put it in the front of your party and battle thousands of trainers or wild Pokemon, OR An easier way is to get the Soothe Bell and just run back and forth until you gain your pokemons Happiness. Every step while holding soothe bell makes it happier. And in HG/SS just walking with you also raises happiness. But don't let it faint or put it in your Box because it will lose all happiness.
He evolves at any level with MAX happiness(you can check your poketech app, the one with hearts over your pokemon. 2 hearts=max happiness.). But, you must level him up with max happiness during the DAYTIME.