For the pet, see  [Happiness].

Until Patch 4.1.0, each Hunter's pet had a happiness rating, also sometimes known as mood. Happy pets dealt 25% more damage than normal, and unhappy pets dealt 25% less. With Patch 4.1.0, happiness was removed. Feeding a pet now heals it but has no other effect.

Happiness affected only hunter pets. Warlock and Death Knight pets are unaffected.

Happiness levels

The happiness of a pet is shown beside the pet's portrait and in his pet paper doll as a small smiley face icon. This icon shows the states of happiness your pet goes through:

Pet happy 19x20.png Happy (Green big smiley face) 
Your pet is perfectly happy with you and the world. In this state, your pet will do 125% of its normal damage to the mob during a fight.
Pet neutral 19x20.png Content (Yellow neutral face) 
Your pet is contented but not really happy. It's worth feeding your pet at this stage to get it back to the Happy state above. Your pet will deal 100% of its damage at this stage.
Pet unhappy 19x20.png Unhappy (Red frowning face) 
Your pet is unhappy and will only do 75% damage during fights.

Each level spans about 350 happiness points. So if your pet just dropped from Happy to Content, it will require food for 350 happiness to get it to maximum happiness.

Gaining happiness

The most basic way to increase the happiness of your pet is by feeding it. One feeding will give the pet 10 bites of food, distributed over 20 seconds. Depending on the quality of the food, each bite will give 8, 17 or 35 happiness, for a total of 80, 170 or 350 per feeding. You can follow the gain in happiness in the combat log. If your pet attacks or is attacked during the feed period, the feeding is interrupted.

However, glyphs and talents allow ways of increasing (or maintaining) a pet's happiness without food.

With the  [Glyph of Mend Pet], the pet gains happiness whenever the [Mend Pet] spell is used. Feeding pets is not necessary with this glyph. A big advantage of the glyph is that it allows the pet to gain happiness while it is in combat. A pet cannot fight and gain happiness via [Feed Pet] because it stops eating when entering combat and won't eat while in combat.

Ferocity pets may learn the [Bloodthirsty] talent, which allows them to gain happiness during combat. This can allow the hunter to get away with not carrying any pet food at all, provided the pet is used in combat frequently enough to allow the talent to proc. Tenacity pets may learn [Guard Dog], which increases happiness whenever [Growl] is used.

As of Patch 4.0.1, pets in the Bear family also have a new ability [Rest] which grants 40 happiness over 12 seconds.

Losing Happiness

Things that make a pet lose happiness are:

Just standing around
Your pet will lose happiness at a steady rate over time. The loss has been reported to be about 50 happiness in 6 minutes.
[Dismiss Pet]
This action makes the pet lose 50 happiness. Removed in patch 4.0.1
It has been reported that the death of a pet causes a loss of 350 happiness.
When you go out of range of your pet (either while it is in combat or you made it Stay) and it disappears, it loses 10 happiness.[citation needed] 
Not helping
There are suggestions that letting a pet kill a mob on its own while not helping will cause the pet to lose happiness.

Mounting up will make your pet disappear, but it will not make it lose any happiness. The pet does not lose happiness over time while the Hunter is mounted.

Patch changes

  • Cataclysm Patch 4.1.0 (2011-04-26): The Happiness/Pet Loyalty System has been removed. Hunters will no longer have to manage Happiness for their pets, and the previous damage bonus for pets being happy will now be baseline for all tamed pets.
  • Wrath of the Lich King Patch 3.0.2 (2008-10-14): The Loyalty property was removed from the game. Happiness now only influences damage done by the pet.