Why do Rudy and Liesel start stealing together?

We need to understand that everything was in short supply, including food. Liesel begins by stealing books which,to her, is as necessary as food. Rudy and Liesel steal together mostly for food. This way of life was a necessity. They only stole from people who could "afford" to lose it. These petty thefts were also a bonding experience for the two. There was an underlying excitement and danger involved where they watched each other's back.