Ego is nothing but thought in my opinion. The thought emanates from self and the thought is 'me', then 'mine' comes. Self is the giver of sense 'I AM' and it's something which is static and never changes.
Ego is cause of separation because, the thoughts are petty and so the estimation of the 'self' is petty, tiny isolated being, which is between myriads of such other isolated beings.
The awakened or enlightened one has a self which is essentially everything and everyone together at the root and hence there are no divisions and separations and no craving to become something else.
Rituals and much of social structure are based on ego. Take for example: Humbleness is mostly cultivated. Parents inculcate in the children a habit of saying 'hello', 'bye', 'good morning' and other such things. This humility is not like fragrance of flower which is for one and all without discrimination. Politicians during their campaigns before elections(especially here, in India) visit poor villages and stay with destitute people in their huts and talk in a very polite tone. This humility is merely a pretense and a gimmick to get votes.
Cultivated humility like many other such traits which are hidden well inside the umbrella of mannerism are ego driven. A person who looks most humble might be most egoistic person pretending to be humble just because he wants people to respect him and his rage and vicious nature become apparent as soon as he gets frustrated by the public.
But our society functions in this way: That is, most of us are very well acquainted with these ego driven mannerisms and it takes infinitesimal sliver of humanity, very acutely conscious who abhor this cultivated mannerism--but on the other hand do not get upset when met with lack of respect and so on.
Take another example: A very popular charitable person spends millions of dollars on charity. He thinks that he is a very good man. He is very eminent and respected by Church as well. He is indeed helping a lot of people. But if he is driven by "I would go to heaven because I am helping the poor" or "See, I am the most eminent man in the world"; his humility is merely a pretense and what he is doing is---creating the bondage of ego which makes him carry this baggage of 'doer-ship' and due to this baggage he attracts circumstances where he has to face disgrace or humiliation and ultimately the shattering of ego.