The private key is the secret. Someone who steals it does not have to break anything. In this case it is similar to a physical lock on a safe: if you lose the

The private key is the secret. Someone who steals it does not have to break anything. In this case it is similar to a physical lock on a safe: if you lose the key to the safe you just have to accept that you no longer can access the contents of the safe. You can have wallets that are protected by multiple keys, but there is not a whole lot of wallet software that does this in an easy to use way right now.