Boredom causes existential growth.
By Rachel's distaste for the pettiness and smallness of her world, she is caused to seek a higher way of life, a life full of risk and passion and mistakes and real success or failure. In a way, the novel could be seen as a coming-of-age story, where the age in question is prime adulthood instead of early adulthood.
Women's relationships are strained.
Rachel is thirsty for company and surrounded by peers, but it's as if the petty high school attitudes remain between them, perhaps for social or cultural reasons, which makes it so that women in this story are particularly lonely. This theme is a central facet of Rachel's relationship to her mother.
Romance isn't a solution to the problems, but it is still valuable.
Instead of Rachel falling in love with someone who answers all her questions and stays around forever, Rachel is seen as the master of her own fate. She realizes that in this case, she could use a little summer fling, but the romance is only a part of her story. The much greater plot points deal with her sense of self as a woman and her relationship to her peers and authority figures. In other words, this is not a story about how marriage saved a woman from her life. It's the story of a woman who knew the limits of love and did it anyway.
Mothers and daughters are often alienated from one another.
Another main part of the story comes from Rachel's existential crisis around watching her mother age and approach death. By showing this relationship as strained and isolating, Rachel is forced to recognize the fact that she and her mother may never reach peace, and Rachel might be asked to move along in life regardless.
