It's sad because we all end up kissing the wrong person goodnight. And not only is it the wrong person, but we know it's the wrong person. And we let them kiss us and tell us we're pretty because dying alone is a fear engrained in the human soul.
We then live hoping to die in the right person's arms, and I guess given the kind of lives we live, that's the best possible way for things to end. This however, is escpesically sad because most of us do not die in the right person's arms, but in no one's arms, despite all the efforts we make for our lives to end differently.