Animals in zoos often live in enclosures or cages that are hundreds of times smaller than their homes in the wild.
Zoos breed animals in order to have babies who will bring a lot of people (with money!) to see them. Older or less popular animals are even sold or shipped off to live in even worse conditions at roadside zoos so that they can be replaced with younger, cuter ones.
Even at the best zoos, being in a cage with people staring at you is nothing like how animals would live in the wild. They are prevented from doing things that are natural and important to them, such as running, roaming, flying, choosing a partner, and being with their families.
Marine parks such as SeaWorld give orcas and dolphins nothing more than what is basically a concrete bathtub to live in. These animals and their families will swim for miles every day in the wild. In captivity, they can only swim in small circles.
Forced to perform for large, noisy crowds, the whales become frustrated, bored, and aggressive. They can become so stressed that they attack each other and humans as well.