By pinning Mary's anger and unhappiness on being lonely, the novel suggests that it's essential for a person to be a part of some community (whether that be composed of humans or animals) in order to be a reasonable member of society. When Mary takes Ben's assessment to heart and asks the robin to be friends with her, it shows that she's learning how to take others seriously and consider what they have to say. The fact that her voice becomes pleasant when she speaks to the robin again speaks to the power of friendship to make a person beautiful, and hints that there is perhaps a more tender side of Mary that is hidden beneath a hardened, angry shell.