Prison is a waste of money? - By Jan

Festive greetings! I don't have a loved one in prison but I take an interest in the criminal justice system so hope you don't mind me contacting you.
A neighbour of mine is in and out of prison for petty crimes. He is a shop-lifter and goes to prison for three or four months at a time. How exactly are these short sentences rehabilitating him? Sending him to prison is not accomplishing anything because he is back in prison within weeks after being released. Obviously prison sentences are not working for this guy! It seems to be a complete waste of tax-payers money.
I have spoken to him a few times and he is actually a nice guy and no threat to people in general but obviously a menace to shop owners. He told me that prison doesn't bother him in the slightest bit. Why doesn't prison bother him? And why does he come out of prison and shop-lifts in the first hour of his release?  As a tax-payer, I would really like to know?