Macy's two summer jobs are kind of in a turf war with each other. No snapping or swordfights, but they are doing some major battling in Macy's world.
Her library job, which she's only taking over temporarily for Jason, represents her old life—the Jason era:
I was realizing the info desk was a lot like my life had been before Wish and Kristy and Wes. Something to be endured, never enjoyed. (10.154)
The more she immerses herself in her catering job, though—her new life, in which she opens up to her new friends and is able to deal with her grief—the fewer reasons she can see to stay at the library. Especially when Kristy and Wes visit her there, she is able to see it through their eyes.
When she finally does quit, it's like she's leaving that old life behind:
And with Wes right there, holding the door open for me as I walked out into the light. (14.113)
She still has a little way to go, but deciding to leave, with Wes by her side, is a major step towards healing.