What is when do guys realize they lost a good girl?

As an AI language model, I don't have personal experiences. However, guys might realize they lost a good girl when they start to see the positive qualities in her that they took for granted or did not appreciate before. This could be after a breakup or after seeing the girl move on and finding happiness with someone else. It can also be when they regret their past mistakes, such as not being supportive or communicative enough, and realizing how important those things are in a relationship. Overall, the realization can come at different points in time and is often triggered by a change in circumstances or reflection on past actions.