Animals aren't totally different than us. We share a lot of similarities biologically, and even in our brain structures. It's safe to assume that herd animals need a decent amount of space to roam around, and that crowding is stressful and possibly causes mental illnesses.

This is even more true of animals like pigs. Imagine dogs living their lives out in kennels. Also consider the change in behavior in zoo animals when their habitats are redesigned to more closely resemble their native habitats.