When someone is imprisoned for a long time, they become “institutionalized.” In other words, they come to like prison. It is their home. It is predictable and safe compared to the outside world.

I worked with kids in residential treatment. Some of them, without parents or so badly abused and neglected, were institutionalized. Yes, it is their home, and for many it is predictable and safer than what they know. It's far too simplistic to say that they "like" it. What is more likely, and what Red speaks to in several parts in the movie, is what happens when the structure that supports you (or you're used to) suddenly drops away? Many in prison or in mental heath facilities did not have the upbringing to instill internal structure, or they're too badly damaged or limited to function independently. A big failing in prison and mental health is the step down process that should be slower to help people re-engage in society.