Why is it considered "stealing" if you are just taking back what was already yours?

Ownership of an item is the job of the court. Even if you are 100% sure it is yours, you can no break into another's property, treasp on another's property, hold them hostage, or threaten them. It is not the stealing back itself that gets you in trouble, but it is that you can't do it without violating other laws in the process.