Understanding Nostalgia in a Short Story

Read the following passage excerpt from a short story:

As the sun set over the quiet town, Sarah stood on her porch, her gaze fixed on the horizon. She remembered the summer days spent running through the fields with her childhood friends and the laughter that echoed in her ears. Those memories filled her with nostalgia, yet an undercurrent of sadness lingered. Change was inevitable, she thought, but it was not the change that frightened her; it was the thought of losing those moments forever.