The title refers to a proverb - or possibly a quote:
Not a door closes but that a window opens somewhere.

Temaris

If it hadn't been for that last attack by the Krenim, nothing would have happened. Harry's life would have gone on just as it always had: playing the clarinet, playing pool, playing ship's ensign. Playing sweet little Harry, Tom's friend.

He sat in Nav forward, reflecting bitterly on everything that had happened. "A curse. I'm a bloody curse," he whispered roughly, ignoring the tears running down his face. "B'Elanna. . ."

<My fault, my fault. If I'd moved quicker, read the display faster, kept the bridge systems optimal, done something, it couldn't have happened. . .>

His mind's eye showed him Tom, weeping over the engineer's fallen form, then, at a word from the Commander - the Captain - returning to the conn, flying them to safety. "Oh Tom, I'm so sorry," he said achingly to the smear of stars outside. "I never wished her dead. . . I never wished her dead. . ."

He said it over again, clinging to the truth of his words. <I wanted him to be happy. If that meant watching him have B'Ela, then that was fine. Truly. Oh, I miss you,> not sure which ghost he was speaking to.

He thought of the beautiful woman who, by now, no longer waited for him, who surely thought him dead, and had mourned him and returned to life without him, and gone on. They had made no promises, and while he knew that one day they had hoped to change that, there was nothing to bind him except his own reluctance to give up the hope of home. Nonetheless, trapped in limbo, he had to endure the lack of closure, feeling as though every choice he made was a betrayal of someone. <Enough. You're alive aren't you?> He remembered the other beautiful woman, lying dead on the floor of a lost starship, her lover crushed by her loss. And now, when really he should be helping to repair the broken ship, all he could do was grieve for more sorrows than he thought he could bear.

"I wish I had died too," he said, reflectively. "It would be so much simpler."

"Coward!" In his thoughts it sounded so like B'Elanna that he whirled, convinced for a moment that he had heard her. There was no one there. He turned again, sitting on the hard floor with his back to the doorway. <Coward? Me? Maybe I am. I don't mean to be. Think of Tom. . .> His thoughts were railroaded by the picture once more, of Tom's heartbreak. <I should go help. . .> he thought firmly, and got to his feet.

He paused by the window, and touched his hand to his lips, and blew gently. "Goodnight Libby. I miss you. Goodnight B'Elanna," he pressed another kiss onto his fingertips, then pressed those fingertips hard against the plasteel, until the nails showed white, as if mere pressure could reach across the divide. He swallowed back a sob. "I'm sorry. I'll wait." <But I'll hope too. . .>