Our core illusion
The ego has been created by lust and desire and its essential nature is desire, and desire is always related to something exterior to ourselves, to something we don't have. We cannot desire what we already have, however we can be afraid to lose what we already posses.
That's why our ego is always compulsively seeking something in the sensory world, something to possess in order to experience pleasure and satisfaction as well something to fulfill it or make it feel complete and worthy.
Since all sense objects devoid of any reality, are impermanent and perishable and since ego's desiring or seeking is a compulsive and repetitive addiction, no matter what we will possess, we will experience and we will achieve, we will always remain at the end with the same painful inner state of emptiness, incompleteness, uneasiness, discontentment, etc.
The seeking of plenitude, happiness, contentment and freedom outside of ourselves, due to non-perception that we already are complete, full, peaceful, blissful and free, it is the core illusion and the womb of all suffering.