To vet frustration and assert some sort of control: to lash out and cause harm due to an inability to harm the ones you truly want to harm.
To feel skilled at tricking or over coming (a form of validation due to a lack of validation or attention.)
Doing it for the thrill, or out of boredom.
To garner attention or earn the respect of someone who would admire that trait.
To rebel against preexisting concepts/ideals. Or to disrespect someone or something you are against.
To become part of a counter culture (ties in with the above. See Abby Hoffman's Steal This Book.)
Um...of course financial strain/desperation.
REVENGE!
Because you're a pirate, and that's what pirates do?
That's all I got.
Edit: comedic idea--person is just super air headed and keeps doing it by accident. When I was in High School I was shopping with my mom and she handed me a box of tea she was going to buy. She takes forever to shop, and I hate shopping so I kind of space out. Anyway, she decided not to get anything so I just followed her out the store...and when we got to the car she's all like--you didn't set that down? I was like "oh shit." We...weren't about to go in and return it, cause who would believe such a stupid accident?
Anyway, if anyone wants to do a comedy of this, it'd be funny to find ways a person keeps accidentally stealing XD