There are a couple of reasons that come readily to mind. The first is that humans - as a species - have a tendency to be arrogant, myopic, self-referential, xenophobic, acquisitive and obsessed with controlling everything around us. The second is that, at least for the more recent span of human history, our activities have been wantonly and overwhelmingly destructive to the natural environments we inhabit. So, from a psychological standpoint, we pretty much have to self-justify these attitudes, proclivities and behaviors by placing ourselves “apart” from everything around us. In a way our species acts a lot like a narcissistic psychopath - believing we are special or better than everything around us, and acting quite hostilely towards all we perceive to be “other” as we manipulate it towards our preferred ends. Of course, humans also have great capacity for empathy, compassion, moral conscience, self-awareness, and a sense of connection with others and the world around us. These more prosocial characteristics allow us to feel awe and reverence for Nature, to accept a more equivalent importance for our species among its functions and process, to see other conscious critters as independent family rather than just facilitators of our self-centered needs, to care about all of the Earth, and to seek harmonious coexistence with our natural birthplace. Unfortunately, as these human qualities are usually at odds with our more psychopathological ones, they have often been suppressed, rejected, belittled or - if they rise up in too great of a concentration or disruption individually or collectively - murdered and enslaved. Thankfully, sometimes our better nature percolates up through a particular zeitgeist, culture or timespan, so that it effectively reins in our pathology. And I think we have potential to continue to blossom our more prosocial selves into prominence over time, so that we become less destructive, and less “apart” in our self-conceptions. But these two facets of human interiority have been battling with each other throughout all of recorded history, and continue to do battle in our current times. The tension never seems to abate for long. One could even say these internal forces are the basis for conceptions of Light and Dark - or good and evil - in many traditions. So the question then becomes: which path will we choose; which wolf will we feed (see Cherokee Legend - Two Wolves)?
My 2 cents.
From Quora post: https://www.quora.com/Although-humans-are-part-of-nature-human-activity-is-often-understood-as-a-separate-category-from-other-natural-phenomena-Why-is-this/answer/T-Collins-Logan