Why is Liesel ashamed of herself?

Liesel is ashamed at herself for not saying thank you.... for not even having the manners to say goodbye.

“You said nothing.” Her head shook vigorously, among the hurried footsteps. “Not a ‘goodbye.’ Not a ‘thank you.’ Not a ‘that’s the most beautiful sight I’ve ever seen.’ Nothing!” Certainly, she was a book thief, but that didn’t mean she should have no manners at all. It didn’t mean she couldn’t be polite.