In one sense it is hard for humans to know who they are because they have been externally hardwired (for survival purposes) at the base brain /mind level (ego)to see themselves as the center of their own universe. In other words nature directs that survival and reproduction (continuation of the species) be the primary mandate of humankind, no different than any other organism.
But what sets us apart from the rest of nature’s organisms is our imagination, as the good doctor has so clearly explained above. Thus when we ask ourselves who am I or what is my purpose, we are leaving the world of objective reality and trying find some rationality that is a nonexistent story line.
In nature’s objective world we only have one purpose and that is to exist and procreate. To that end we seek out advantage at every opportunity, even though for social reasons we often feel compelled to hide from others and ourselves our true egocentric objectives.