What is the theme in The Second Summer of the Sisterhood by Ann Brashares?

The Second Summer of the Sisterhood is first and foremost a coming-of-age novel because it revolves around four teenage girls who experience significant growth during the course of one summer.

When the summer begins, Lena is unsure of her feelings for Kostos, the young man she met in Greece the previous year. Although she cared deeply about him, she cut off contact with him several months earlier, largely because she felt unable to handle her feelings for him, but their romance is rekindled when he arrives in her hometown for a visit.