Animals are, for the most part, happy. Because they 'know' very, very little. They have zero understanding of the way the world works, all they know is the way THEY work.
Intelligence, judgement, and wisdom will not make you happy. Even the Zen Yogis define Happiness as nonexistence.

Humans, however, were not designed to be happy. we were designed to succeed and strive to excel past those animals that are always happy. Intelligence exists to allow us to profit, as a species, from our unhappiness.

God, or nature (If you are an atheism cultist) has custom-fitted us with a sort of permanent unhappiness… However, as part of that baggage, we ALSO can understand how the world works, and seek to find out more about how it works.

Content is drawn from understanding that some things are simply vastly more important than 'happiness'.