How did Annemarie and Ellen's friendship change?

In Chapter Twelve the girls' friendship changes because it is time for Ellen to leave. Ellen promises to come back, but no one knows if that promise can be kept. Ellen's departure leaves Annemarie alone.

"I'll come back someday," Ellen whispered fiercely. "I promise."

"I know you will," Annemarie whispered back, holding her friend tightly. Then they were gone, Mama and the Rosens. Annemarie was alone. She went into the house, crying suddenly, and closed the door against the night.