Signs of criticism
Monica's character is formed in this play by motif. She pesters her waitstaff. She mentions Claire's weight struggle. She even tells her sister to shut up. Clearly she is using criticism as a tool to establish herself. These signs of criticism are projected; Monica struggles with her self-esteem and makes others the victim of that self-esteem by sharing a hyper-critical point of view.
Sisters as a symbol
Since Cain and Abel, sibling stories have been well-rehearsed in human literature. This dyad represents too hypothetical responses to the same problems. While there are clearly systemic issues plaguing the woman (for instance, they are grown adults who have never had an orgasm once for fear of shame). Their differing points of view sometimes pit them against each other, so they represent the community of women and the way women relate to each other.
Masturbation as a symbol of happiness
Claire is quiet, but assertive. This is evidenced in her willingness to explore her own body. Interesting that she has not done that before—perhaps she felt her body was the property of some future husband? Her celebrating her autonomy and sovereignty is seen in the pleasure and self-indulgence of masturbation. She is willing to ask the difficult questions about shame and happiness.
Norman Rockwell
The Norman Rockwell Museum is exactly what it sounds like. As a whole, Rockwell's work perfectly captured the nostalgia of Americana, of the American Dream. He also captured profound sadness and loneliness, so the sisters discussion of his art is itself an artistic symbol of their participation in their own culture. They have a chance to scrutinize the assumptions about what happiness looks like in America.
The generous tip
The play ends on a nice note: the girls exchange difficult words, telling each other what they still dislike about the other (interesting that the story pits them against each other this way: a smart reader could easily argue this scene is an argument for feminism). In the end, when they make peace and Monica leaves a generous tip, we see why they were able to make peace: they love each other, and although they are different and sometimes competitive, their respect for the other is more important than their impulsive to criticize the other. It is a symbolic reminder to support one another.
