The common enemy of all religious disciplines is selfishness of mind. For it is just this which causes ignorance, anger and passion, which are at the root of all the troubles of the world.

Desires can be either negative or positive. If I desire to acquire something for myself--let's say I desire good health when I am ill, or a bowl of rice when I am hungry--such a desire is perfectly justified. The same applies to selfishness, which can be either negative or positive.
In most cases, asserting oneself only leads to disappointment, or to conflict with other egos that feel as exclusively about their existence as we do about our own. This is especially true when a strongly developed ego indulges in capricious or demanding behavior. The illusion of having a permanent self is a secret danger that stalks us all: "I want this," "I want that." It can even lead us to kill. Excessive selfishness leads to uncontrollable perversions, which always end badly. But on the other hand, a firm confident sense of self can be a very positive element. Without a strong sense of self, that is, of one's skills, potential, and convictions, nobody can take on significant responsibilities. Responsibility requires true self-confidence. How could a mother without hands save her child from the river?
The Dalai Lama's Little Book of Inner Peace

Ego is the absence of true knowledge of who we really are, together with its result: a doomed clutching on, at all costs, to a cobbled together and makeshift image of ourselves, an inevitably chameleon charlatan self that keeps changing, and has to, to keep alive the fiction of its existence.
In Tibetan, ego is called dakdzin , which means “grasping to a self.” Ego is then defined as incessant movements of grasping at a delusory notion of “I” and “mine,” self and other, and all the concepts, ideas, desires, and activities that will sustain that false construction.
Such grasping is futile from the start and condemned to frustration, for there is no basis or truth in it, and what we are grasping at is by its very nature ungraspable. The fact that we need to grasp at all and to go on grasping shows that in the depths of our being we know that the self doesn’t inherently exist. From this secret, unnerving knowledge spring all our fundamental insecurities and fears.

Lifetimes of ignorance have brought us to identify the whole of our being with ego. Its greatest triumph is to inveigle us into believing its best interests are our best interests, and even into identifying our very survival with its own. This is a savage irony, considering that ego and its grasping are at the root of all our suffering.
Yet, ego is so terribly convincing, and we have been its dupe for so long, that the thought that we might ever become egoless terrifies us. To be egoless, ego whispers to us, is to lose all the rich romance of being human, to be reduced to a colorless robot or a brain-dead vegetable.

To end the bizarre tyranny of ego is why we take the spiritual path, but the resourcefulness of ego is almost infinite, and it can at every stage sabotage and pervert our desire to be free of it. The truth is simple, and the teachings are extremely clear; but I have seen again and again, with great sadness, that as soon as they begin to touch and move us, ego tries to complicate them, because it knows it is fundamentally threatened.
However hard ego may try to sabotage the spiritual path, if you really continue on it, and work deeply with the practice of meditation, you will begin slowly to realize just how gulled you have been by ego’s promises: false hopes and false fears. Slowly you begin to understand that both hope and fear are enemies of your peace of mind; hopes deceive you, and leave you empty and disappointed, and fears paralyze you in the narrow cell of your false identity. You begin to see also just how all-encompassing the sway of ego has been over your mind, and in the space of freedom opened up by meditation, when you are momentarily released from grasping, you glimpse the exhilarating spaciousness of your true nature.

Two people have been living in you all your life. One is the ego, garrulous, demanding, hysterical, calculating; the other is the hidden spiritual being, whose still voice of wisdom you have only rarely heard or attended to. As you listen more and more to the teachings, contemplate them, and integrate them into your life, your inner voice, your innate wisdom of discernment, what we call in Buddhism “discriminating awareness,” is awakened and strengthened, and you begin to distinguish between its guidance and the various clamorous and enthralling voices of ego. The memory of your real nature, with all its splendor and confidence, begins to return to you.
You will find, in fact, that you have uncovered in yourself your own wise guide, and as the voice of your wise guide, or discriminating awareness, grows stronger and clearer, you will start to distinguish between its truth and the various deceptions of the ego, and you will be able to listen to it with discernment and confidence.