[He stiffens at the laughter, and chooses to double down on what he knows to be true. He'd never be able to bear it otherwise.]

I believe it because it's true. Klaus isn't who he used to be. He's repented for his mistakes and has no intention to repeat them. He's better.

[And what he needs most is someone to believe in him. Apparently even his girlfriend can't accomplish that much for him at the moment.]