People who don't think that an animal can experience happiness have one major point in their argument's favor: There's no proof that animals can be happy. Any evidence of happiness in the animal kingdom -- for example, a goat prancing or meerkats playing -- is simply anecdotal, which doesn't stand as evidence according to the scientific method.
What's more, it's possible that most animals (with the possible exception of some other primates, dolphins and elephants), plainly lack the capability to experience an emotion like happiness. Emotion requires three processes: a physiological response to a certain stimulus, an outward expression of emotion and an analysis of that emotion [source: Griggs]. While animals like rats can experience the first two (perhaps in the form of an involuntary fear response), they haven't been shown to possess the reasoning skills needed to analyze how that makes them feel at that very moment. This reasoning and analysis forms the basis of higher emotions.
Critics of animal happiness dismiss the idea that animals can experience happiness as anthropomorphism. This is the tendency among humans to attribute human characteristics to nonhuman beings and objects. We are simply applying things we're familiar with, like emotions, to these things to help us understand our surroundings. It's a lot easier to explain a prancing goat as being "happy" than it is to study the behavior further and determine that the dance is part of a mating ritual. Anthropomorphizing is the point at which human curiosity meets human laziness.
There are plenty of examples of humans anthropomorphizing animals. One great example is the story of Hachiko. This dog, which lived in Japan in the 1930s, accompanied his master to the train station each day and was there, waiting for him, when he returned home. When his master died while he was away and never returned, the loyal Hachiko spent the rest of his years patiently returning to the train station each afternoon to wait for his master's return.
In that brief description, at least two acts of anthropomorphizing took place -- Hachiko was described as patient and loyal, two human traits. While we can definitively say that Hachiko accompanied his master to the train station and returned daily to greet him once more, even after the man had died, we cannot say that Hachiko actually felt loyalty or patience. Those, like happiness, are subjective experiences we cannot prove that an animal is capable of feeling.
This is but one side of the argument, however. What if animals can, in fact, experience happiness?