“I believe that the nature of man is a contradiction rooted in the conditions of human existence that requires a search for solutions, which in their turn create new contradictions and now the need for answers.
I believe that every answer to these contradictions can really satisfy the condition of helping man to overcome the sense of separation and to achieve a sense of agreement, of unity, and of belonging.
I believe that in every answer to these contradictions, man has the possibility of choosing only between going forward or going back; these choices, which are translated into specific actions, are means toward the regressing or toward the progressing of the humanity that is in us.”

The portion of this statement, "Love is the only sane and satisfactory answer to the problem of human existence" has been widely quoted alone, resulting in a less reserved expression, and sometimes the portion following it has been as well: "Any society which excludes, relatively, the development of love, must in the long run perish of its own contradiction with the basic necessities of human nature."
The Art of Loving (1956)
Context: Our society is run by a managerial bureaucracy, by professional politicians; people are motivated by mass suggestion, their aim is producing more and consuming more, as purposes in themselves. All activities are subordinated to economic goals, means have become ends; man is an automaton — well fed, well clad, but without any ultimate concern for that which is his peculiarly human quality and function. If man is to be able to love, he must be put in his supreme place. The economic machine must serve him, rather than he serve it. He must be enabled to share experience, to share work, rather than, at best, share in profits. Society must be organized in such a way that man's social, loving nature is not separated from his social existence, but becomes one with it. If it is true, as I have tried to show, that love is the only sane and satisfactory answer to the problem of human existence, then any society which excludes, relatively, the development of love, must in the long run perish of its own contradiction with the basic necessities of human nature. <!-- p. 111 - 112

On the Correct Handling of Contradictions Among the People
Original: (zh-CN) 马克思主义的哲学认为，对立统一规律是宇宙的根本规律。这个规律，不论在自然界、人类社会和人们的思想中，都是普遍存在的。矛盾着的对立面又统一，又斗争，由此推动事物的运动和变化。矛盾是普遍存在的，不过按事物的性质不同，矛盾的性质也就不同。对于任何一个具体的事物说来，对立的统一是有条件的、暂时的、过渡的，因而是相对的，对立的斗争则是绝对的。

Materialism and Empirio-Criticism (1908)

Political Theology (1922), Ch. 4 : On the Counterrevolutionary Philosophy of the State

Human Nature and Social Theory (1969)
Context: One will be conducive to cooperation and solidarity another social structure to competition, suspiciousness, avarice; another to child-like receptiveness, another to destructive aggressiveness. All empirical forms or human needs and drives have to be understood as results of the social practice (in the last analysis based on the productive forces, class structure, etc., etc.) but they all have to fulfill the functions which are inherent in man’s nature in general, and that is to permit him to relate himself to others and share a common frame of reference, etc. The existential contradiction within man (to which I would now add also the contradiction between limitations which reality imposes on his life, and the virtually limitless imagination which his brain permits him to follow) is what I believe to be one of the motives of psychological and social dynamics. Man can never stand still. He must find solutions to this contradiction, and ever better solutions to the extent to which reality enables him.
The question then arises whether there is an optimal solution which can be inferred from man’s nature, and which constitutes a potential tendency in man. I believe that such optimal solutions can be inferred from the nature of man, and I have recently found it quite useful to think in terms of what in sociology and economy is now often called »system analysis«. One might start with the idea, in the first place, that human personality — just like society — is a system, that is to say, that each part depends on every other, and no part can be changed unless all or most other parts are also changed. A system is better than chaos. If a society system disintegrates or is destroyed by blows from the outside the society ends in chaos, and a completely new society is built upon its ruins, often using the elements of the destroyed system to build the new. That has happened many times in history. But, what also happens is that the society is not simply destroyed but that the system is changed, and a new system emerges which can be considered to be a transformation of the old one.

Source: How to Read and Why

Source: The Republic of Wine (1992), Chapter Four, Section 3

Source: The Art of the Commonplace: The Agrarian Essays

On the Correct Handling of Contradictions Among the People
Original: (zh-CN) 在一九四二年，我们曾经把解决人民内部矛盾的这种民主的方法，具体化为一个公式，叫做“团结——批评——团结”。讲详细一点，就是从团结的愿望出发，经过批评或者斗争使矛盾得到解决，从而在新的基础上达到新的团结。按照我们的经验，这是解决人民内部矛盾的一个正确的方法。