There's just a LOT of theft in Barcelona. Pretty much every single day, somebody in my university gets their phone/bag/wallet stolen, and I literally mean every single day. It's so common here that people don't even talk about it anymore.
I think it stems from the fact that the law here almost encourages petty theft (under 300 euro) and I'm guessing these guys eventually go on to larger things. I just heard a couple of days back that a few of my friends (3 girls who live together) got robbed while they were at home. There's very little violent crime here, but jeez there's a lot of theft.