Long post ahead. TL;DR I stole too and don't know why.
I am really interested to see what answers you get to this, as I had a problem with it in childhood. Childhood was unhappy from age 2 or 3 onwards, but the stealing episode was after I switched schools, aged 8. This is the same time my stepdad appeared, too.
(The school was good and I liked my stepdad, fwiw, but both were major changes.)
I have no idea why I did it. It started out sneaky and frustrated at being denied things even though they were tiny and cheap.
I stole from shops. First, an eraser. I hid it at home and made excuses as to where it had come from when it was found. Another time: a little clip-on bear. There was a GIANT BUCKET of these things, little clip-on bears, for 50p each. I already had one at home; I wanted it to have a friend. I was exasperated that Nmum was so tight she wouldn't get me a 50p bear, so I slipped one when no one was looking, because giant bucket -1 still equals giant bucket.
When Nmum saw me playing with two bears, she gave me all this BS: "How are the people at the shop going to feel? Really sad. They'll realise a bear is missing and be so sad." I remember fixing her with a WTF face. I knew damn well the shop staff wouldn't even have noticed. I knew they weren't going to sit around going boo-hoo and dabbing their eyes. I realised that the adults raising me were full of shit.
What disturbs me most in my memory is stealing from Nmum's friends. I relished doing that. Her friends were grumpy, miserable old women and I didn't respect or like them. The one I stole most from had a serious victim mentality and was always down and whining. I remember walking around upstairs in her house while the adults talked downstairs, saying "Say goodbye to this... and this..." as I put her jewellery in my pocket. I gave it all away in the school playground. Giving it away might have been altruistic, but the theft was cold-blooded. I was 8.
When Nmum found out I'd stolen from her friend, she told me she was going to kill me, and I thought it was true. I assumed that as she'd created me, she had the right to kill me as well. I got the full Spanish Inquisition and remember sitting on this wooden stool wondering how I would be killed, and if it would hurt.
Why? Why the stealing? Why was I so cold and calculating? I have no idea.
I've never told this to anyone.