Love has its own time, its own seasons, and its own reasons for coming and going. You cannot bribe it or coerce it, or reason it into staying. You can only embrace it when it arrives and give it away when it comes to you.
But if it choose to leave from your heart or from the heart of your lover, there is nothing you can do and there is nothing you should do.
Love always has been and always will be a mystery. Be glad that it came to live for a moment in your life. If you keep your heart open, it will come again.