I don't get it

My son is home from college for the summer and will be working 40 hours per week at Ben & Jerry's where he's the store manager. He plans on living off campus in September so he needs all the $$ he can get.

He's been home since May 14 but still unpacking his stuff (which is covering his room, so much so that he's sleeping in his brother's bedroom. It's really the guest room I guess since Kevin doesn't live here anymore but I still call it his room.) As he was packing up his dorm room he noticed things missing. When he came home in December he forgot his electric razor at school. When he returned it was gone. Coincidentally a few weeks previously the college placed a new student in their suite because he didn't get along with his current roommate. When he came back from Christmas break Billy was gone, transferred to another school.

Fast forward to the end of the year and packing up. Turns out he's missing 3 PS2 games, an X-Box game and a bunch of DVDs. His roommate was also missing some games, a guitar hero controller and a few other things. The razor Cam took responsibility for, he left it in the suite's common bathroom but the things missing from his room, well his roommate left for Christmas break after Cam and didn't bother to lock their room.

I don't get why someone would steal someone else's stuff. Wait, I suppose I do get it. Some people see something they want and don't want to work for it, or maybe they don't have the money for it or they're just opportunistic. The bad thing about it is that Billy probably didn't think twice or feel any guilt. It's a bitter pill to swallow when you wouldn't think of doing that but someone does it to you.