Why did Zhu particularly admire Mencius?

In the Southern Song Dynasty, Zhu, who had always criticized Wang Anshi, also praised Mencius, pointing out that "Mencius deserved it and was invited". Zhu was a master of Neo-Confucianism in the Song Dynasty, and his theory merged with Cheng Cheng's "Luo Xue" and became "Zhu Cheng Neo-Confucianism". Zhu's study of Mencius can be said to be a milestone in the history of Mencius' study. His main works are: Meng Ziji, On the Essence of Mencius, Problems and Discrimination of Mencius, Four Books or Problems, etc. And there are countless comments about Mencius in Zhuzi's literary genre and Zhu Wen's official documents. In Zhu's theory, the ingenious combination of Mencius' thought and Buddhism's "mind" fully embodies the tendency of Confucianism in Song Dynasty to actively accept and absorb Buddhist culture in order to transform and develop China's traditional culture. For example, Zhu said in the sixth chapter of The Analects: "Heart and benevolence are the same thing. If separated by selfish desires, the heart will violate benevolence, but they are two different things. If there is no selfish desire, then heart and benevolence will not run counter to each other and synthesize one thing. The heart is still bright in the mirror, but it was originally bright in the mirror, and it was covered with dirt, so it is unknown. If the dust settles, Kagami Akira will cease to exist. " Although it is The Analects of Confucius, it embodies Mencius' thought of "good nature". The last two sentences are in line with the famous sentence of Shen Xiu, one of the representatives of Zen Buddhism: "The body is a bodhi tree, and the heart is a mirror. Always wipe it and don't make dust." Echo each other. Zhu's combination of Mencius and Buddhism can be seen. From Zhu to-Continuation of Mencius' Thought and Promotion of Philosophy of Law from Zhu to-Continuation of Mencius' Thought and Promotion of Neo-Confucianism's Core is "Reason". "Reason" is the root of all things and the supreme authority in the universe. "Reason" is the highest, but it is abstract. Zhu classified the "conscience" and "original heart" put forward in Mencius' theory of goodness of human nature into the category of "reason", and thought that "human nature" was a part of "reason" and was its personification, and "reason" existed before "human nature". The reason why "human nature is good" is because of the existence of "reason". However, "reason" is an object that exists independently of "human nature". In order to achieve "reason", it is necessary to "know everything". "Understanding knowledge by observing objective things" emphasizes understanding "reason", and the method of academic research is external. Zhu proposed in "University Chapters and Sentences": "The so-called governor knows what he knows, and those who want to make me know what he says are poor in reason. To cover the soul of the human heart is to know everything, and everything in the world is to be reasonable. Because reason is not poor, its knowledge is inexhaustible. Starting from the university, scholars must be everything in the world, and will benefit the poor because of their known principles, thus reaching the ultimate. As for the hard work for a long time, once it is suddenly clear, everything is fine and thick, and the heart is of great use, which is unknown. This is called a matter, and this is called knowing. " Source: Western Social Sciences