The statement could be interpreted in a few different ways. One interpretation is that humans often refuse to accept their basic nature and the realities of the human condition, such as mortality and the lack of inherent meaning in life, and instead spend much of their lives trying to transcend these realities or seeking external sources of meaning or validation.
Another interpretation is that humans often strive to be more than what they are, constantly seeking to improve or change themselves, often driven by societal expectations or personal dissatisfaction. This constant striving and refusal to accept oneself as is can lead to internal conflict and existential anguish.
Yet another interpretation is that humans, unlike other animals, are capable of self-reflection and self-awareness, which allows them to question their own nature and existence. This self-reflection, however, can also lead to a refusal or an inability to accept oneself as one is, resulting in a perpetual struggle with one’s own identity and place in the world.
In all these interpretations, the core idea is that humans are unique in their capacity to question, challenge, and often reject their own nature, leading to a variety of existential conflicts and struggles.