We don't know anything about Rosa's past, but what are some possible reasons she is mean?

Rosa has a dirty mouth, and does not easily express her love of others. She is more than a little abrasive and insults those she loves, particularly her husband. We know nothing about her past, so judging from her present I'm going to guess that she had an abusive childhood and doesn't really know how to be loving. Max for whatever reason, comes on the scene and we see her as beautiful for the first time through her care of him. Maybe she lost her dad as a young girl, and maybe that's what made her own life so difficult. In the end, she becomes an endearing figure all around.