Throughout his life Krishnamurti pointed out the fact as he saw it that the observer is the observed: that is the observer is not separate from what is observed, is not different to it, but in fact is it. Ordinarily it is the case that the human brain exhibits behaviour in line with the notion that what is observed is separate from the observer who is observing or experiencing it. This by and large is where the brain of humankind is to be found.
Two factors appear to govern this as a reality. One is the sense that this experience is immediate, and the second is that it is utterly compelling in nature, and so is never questioned or doubted. That is, there is no sense of anything existing which may be driving this state of affairs into being each moment, creating a self with content it considers itself separate from, as a curator of it.
The statement, the observer is the observed, casts doubt on this as a reality, and as such carries profound implications and ramifications for the so-called individual and its worldview. It has the potential to shatter any certainty the brain is reliant upon, which can in turn arouse great fear and anxiety. There is a reason the brain struggles with the actuality of the observer is the observed, since there is much at stake for the self.
It may be helpful to consider in the first instance what is involved in making this sense of separation as observer and observed so immediate and compelling, as well as what there is invested in this worldview.