The quote comes from the seventh chapter of "Flowers in the Mirror": "It is a pity that a virgin has failed in his ambition, but a blessing in disguise is not a blessing, how can you know that it is not a blessing?"
How can you know that it is not a blessing? It is a metaphor that although you suffer a loss for a while, you can gain benefits from it. It also means that bad things can turn into good things under certain conditions, and vice versa. To describe a person's mentality, one must be optimistic. Everything has two sides, and the bad side may turn into a good side.