Understanding Main Idea in a Narrative

Read the following excerpt from a fictional narrative:

"As the sun set beyond the mountains, casting a golden glow over the fields, Emily realized that the summer she thought would never end was slipping away. She recalled the laughter shared with friends, the untamed nights spent by the lake, and the comforting presence of her family. Each memory fueled a bittersweet sense of nostalgia; she understood that while seasons changed, the moments they encapsulated would remain a part of her forever."

Based on this excerpt, what is the main idea of the passage?