Bitcoin is simply digital property. And like real world property, you can mess up and lose it irrevocably.
If you give your gold to the wrong person or drop it to the bottom of the sea, you're probably not getting it back.
If you under fund your military, and someone else comes in and takes control of your land, you're probably not getting it back.
If you send your bitcoin to the wrong address, or mismanage your private keys such that they are stolen, you're probably not getting it back.