Jack had severe kleptomania as a kid, and it’s something he never really grew out of. Nicking little shiny things, coins from distant planets, sparkly rocks, marbles, anything he thought he could get away with. He kept them all in a little box beneath his floor boards, hidden away where his grandmother wouldn’t find them. They never had any real sentimental value, but they made him feel like someone important. Someone who could afford to have a bunch of useless stuff just for the heck of it. 

He doesn’t do it as much nowadays, but he did have quite a relapse while he was dating Moxxi. He’d always play dumb whenever various bits of her costume jewelry went missing. (It was always the cheap plastic stuff; Moxxi always kept her real jewels hidden safely away. You could never be too careful on Pandora.) But when Jack’s feeling particularly antsy, he’ll swipe whatever he can get his hands on (a pack of gum, a rogue bullet from Nisha’s dresser, some sentimental trinket from the desk of an employee), even when he’s got enough money to buy a diamond pony. He never keeps any of it, but it scratches an itch somewhere in his conscious. Most of the time he doesn’t even notice he’s doing it.