i found this fascinating:
Most importantly from a social perspective, the pursuit of happiness is associated with selfish behavior — being, as mentioned, a “taker” rather than a “giver.” The psychologists give an evolutionary explanation for this: happiness is about drive reduction. If you have a need or a desire — like hunger — you satisfy it, and that makes you happy. People become happy, in other words, when they get what they want. Humans, then, are not the only ones who can feel happy. Animals have needs and drives, too, and when those drives are satisfied, animals also feel happy, the researchers point out.