In The Book Thief, are seemingly angry people simply protecting themselves from the hurt which inevitable lies in human relationships?

Are you referring to Rosa as being grumpy because she is merely protecting herself emotionally?

Rosa is a tough woman, but she is also filled with love. She's had other foster children; she works hard, and she demands respect. She will never be the pushover Hans is, and yet don't dismiss the fact that she comes to love both Liesel and Max...... she even nurses Max through his illness. It might be possible that Rosa's anger really is a form of protection..... and that the wall she puts up is to protect herself from loving too much.