Why can't Lucy form a lasting attachment with Peggy? Why does their friendship wane?

Lucy's relationship with her best friend Peggy is no longer progressing forward. At one time, Lucy felt that they were relating well, despite their differences: "The funny thing was that Peggy and I were not alike, either, but that is just what we liked about each other; what we didn't have in common were things we approved of anyway" (61). They often would hang out together, and after seeing Peggy smoke, Lucy too began the habit. Now, however, Lucy notes that "the small differences between us were beginning to loom, sometimes becoming the only thing that mattered--like a grain of sand in the eye" (94). Peggy does not read or go to the museum, cultural experiences that Lucy has become passionate about.

Partly to subvert her attachment to Peggy, Lucy starts dating Paul, a struggling artist whom Peggy has expressly forbidden Lucy to see.