They're not trying to destroy your thesis, they're trying to make sure it's complete and sccurate. In theory, the only person who knows the material is you, so they can't just ask anyone else.
I never really understood why they call it a defense. Nobody's going in there trying to insult or fight it.
How science often works is you try to disprove things, and if you can't, you accept them as likely to be true. So, to show that the thesis is complete and accurate, they're trying to find places where it's incomplete or inaccurate. In the defense, your job is to defend against these attempts.