But lamenting the end of a good time is incredibly unmindful act. It means you're consumed with the past rather than wallowing in the now.
It's especially bad when you feel nostalgic about something that isn't even over yet -- spending the last moments of an experience worrying about the end instead of relishing the remainder.
Nostalgia for the past, that's real. Sadness when you say goodbye to a loved one, that's honest and healthy. But dwelling on them, letting those feelings distract you from all the rest that life has to offer, that's not healthy.
When you choose to end a great experience with wistfulness, you do the experience an injustice. It may have been an awesome vacation/summer/holiday/party/dinner, but turning to sadness at the end takes you away from all the awesome feelings that made it so great.
Replacing the memory of joy with melancholy is no way to honor the past.
Instead, think back fondly. Remember the laughter, the lessons, the friendship. Plan a return trip if you want. But then come back to now.
Right now is the best time because right now is when all the moments of the past join together. Every fragment of time has coalesced to make you the amazing person you are today and to set you up for all the possibilities of the future.
When you honor those moments, when you think of them fondly instead of wistfully, that's when they shine.
Don't look back at past moments with a tear, look forward with a smile, and use every one of those moments to create awesome new ones.