Cassandra Clare said this quote

He remembered Tessa weeping in his arms in Paris, and thinking that he had never known the loss she felt, because he had never loved like she had, and that he was afraid that someday he would, and like Tessa he would lose his mortal love. And that it was better to be the one who died than the one who lived on. He had dismissed that, later, as a morbid fantasy, and had not remembered it again until Alec.