Why are goodbyes so unbearable?

She rests in his embrace for as long as she can. She can't let go. When she finally pulls away tears are streaking down her cheeks. Upon seeing her tears, his eyes well up and he brushes hers away. He traces the contour of her face with his fingers, all the while gazing into her sad green eyes. He pulls her close again and holds her tight for a few short moments that they both know are the end of their precious time together. She cries into his chest; his tears sparkle in her hair. She looks up into his face for one last time and whispers in all the voice that she can find, "I love you..." His voice is held back by tears and he leans in and kisses her, his lips sealing the love with hers. " I love you too..." and with that they let go, their hands holding on until the last minute when he breaks away and walks out the door.