Earlier quoted context omitted.
This brings up an excellent side point that I taught my children... that stealing is not taking something that belongs to someone else, it is taking something that does not belong to you. It is a subtle, but important, distiction that is often used as a rationalization. Edit: This really belongs on the child comment.
That's not really true, though. If I'm walking in a forest and I pick up an apple, prior to my picking it up, it did not belong to me. Upon picking it up, it now belongs to me. You can claim things that did not belong to you prior to said claiming. I get what you're saying but it only applies in situations where everything is already owned by someone.
My take on Tactic's explanation of stealing is that the guilt you should feel from stealing should also be from having taken something that isn't yours from someone, instead of just the guilt of depriving someone from the thing you stole. Although both perspectives are good to have, taking something that isn't yours is a better framework to think in so you can avoid the grey areas of stealing.