Why is it said that "after three days of separation, we treat each other even more with admiration"

Lü Meng of Soochow during the Three Kingdoms era can be said to be a learned and talented man. After Zhou Yu's death, he succeeded him as the governor of Soochow. Guan Yu, who planned to defeat the Shu Han, sent his general Pan Zhang to kill Guan Yu and died soon after. Lu Meng was originally a person who did not do his job properly and was unwilling to study hard, so he had little knowledge. When Lu Su met him, he felt that he had nothing to offer. Later, when Lu Su met him again, he saw that he was completely different from before. He was so powerful. When he talked about military issues with him, he seemed very knowledgeable, which made Lu Su feel Very surprised. Then he smiled and joked to his superiors: "Now that you are so knowledgeable, brave and resourceful, you are no longer the Amen of Wu." Lu Meng replied: "Three days after we say goodbye, you should look at me differently. "Take care of me!" The original text of Lu Meng's words is "Three days of separation, treat each other with admiration." Later people used the phrase "Three days of separation" to praise people for making rapid progress soon after leaving.