Shakespeare is widely acknowledged as the greatest writer of the English language, the person most able to express insights powerfully and memorably. One of the best things he wrote is that deceit, pretense, play-acting, role-playing is at the core of human nature.
Of course, since we're so sure we know who we are, we cannot possibly hear even the great virtuoso of the English language when he tries to tell us as baldly and eloquently as possible that we're wrong about who we are, that we're not who we think we are, that we're people pretending to be who we think we are. As Simon and Garfunkel wrote, A man hears what he wants to hear and disregards the rest, or as William Blake put it so pithily long before As a man is, so he sees.
So there's another part of our answer - we can be wrong about anything. We are even capable of being wrong about the most central issues in our lives (like who we are) and yet be completely convinced that we're right. The idea that we're capable of meaningful objectivity about the things that matter is refuted even before we open our mouths, just by the things we imagine about ourselves - like who we are. The truth is a foreign language to us; we speak something else. As the Christian apostle Paul put it rather poetically in his First Epistle to the Corinthians, we see through a glass, darkly. As Heraclitus put it rather philosophically in his lost work On Nature, Most people do not take heed of the things they encounter, nor do they grasp them even when they have learned about them, although they suppose they do—and, more bluntly, Human nature has no real understanding; only the divine nature has it.
Our essential subjectivity, our incapacity to recognize, observe, or understand the truth shows clearly in our efforts to be objective—we can collect "facts" (Hegel: There is precisely no such thing as a fact.), but nothing ensures we collect the right facts, nor that we interpret them correctly. Oh, does E equal MC squared? Surely that must be true so that I can find a new way to dominate the other monkeys. Obviously, the profound truths of the cosmos exist so we can pursue the pettiest of aims with greater efficiency! The twentieth century all by itself is sufficient to disprove any possibility that human nature is essentially capable of objectivity.
Too often we choose between either lying to ourselves and others about our biases and pretending to be objective or else abandoning that pretense and openly embracing superstition, bigotry, and other forms of extreme irrationalism - which is just another forms of pretense. There are better choices open to us, if we have the courage to take them. At our best we could openly acknowledge and accept our subjectivity and try to approximate objectivity not through pretense but through taking our biases into account, factoring them into our decisions - and then embrace humility and acknowledge that our ideas are tentative, that we do not really know anything but are guessing as best we can.
To get closer to the truth, we first have to admit that we don't already know the truth. If knowledge is power, then for human beings humility is the root of all real power.