I've been in lots of prisons as a professional.
It's certainly not a bed of roses all the time, but in some cases, the inmates might think 'it's not so bad really' especially if on the outside they have issues with debt, lack of money, homelessness etc. At least in prison you get a bed, medical attention and adequate food.
But there will be fights, bullying, intimidation etc. And like every other public service, resources are massively stretched, so that care and rehabilitation isn't what it should be.
In women's prisons especially, the inmates really suffer due to being separated from their DCs and worrying about if they are being looked after properly on the outside. There is a problem with self harming and suicide.
Both Vicky Pryce and Jeffrey Archer's books are an interesting insight into life in prison and the sorts of people who are in there and the crimes they have committed, which can be everything from hardened repeat criminals and truely evil people to otherwise good citizens who made a mistake, got tricked into crime by other people, or got caught doing things that many of us will have done without thinking too much about it.
For example, years ago an old boyfriend asked me to pick him up and give him a lift somewhere. When I picked him up, his mate, who I vaguely knew, was with him and they put some stuff in the boot of my car. We drove to another mates and some of the stuff was removed but some of it stayed there for a day or two, before boyfriend removed it and left it in his flat. What I didn't know was that stuff had been stolen, so obviously if I had been caught with it, I had been handling stolen goods, and that is the sort of crime that is mentioned in Vicky Pryce's book.
Also things that happen when people get a bit drunk and silly on a night out. You only need someone to accidentally get hurt or make a complaint and people are looking at a jail sentence.