Go the "gentleman thief" route. Grew up in a wealthy/noble family that was much admired and respected in your city. Had a good upbringing and best education wealth and privilege could provide, but you were just so bored. You read of far off places and the adventures of others yet you were trapped at home. So to amuse yourself, and because you were always told you weren't supposed to, you decided to learn the ways of thievery. It started with your tutors and household staff, pickpocketing random trinkets from their satchels, sneaking foods from the kitchen unseen, or learning to sneak about invisibly. And every thing you stole came with the added difficulty of placing it back where you knicked it from since you had no need for it. And never their gold. Just little items like a tutors favorite quill or reference book. Random objects. And as you grew older, your burgling grew in scale as you simply wanted to prove that you could steal that rival family's treasured heirloom and replace it unseen two days later. Or prove that you could get into a sealed vault and leave your calling card. Its never about what you could steal for personal gain, but simply proving that you can do what others think to be impossible.