Egocentrism is a natural component of childhood as most children must be egocentric in order to survive. Their brains have not yet developed into having an understanding that others exist in the world with needs that equal their own. If children grow to be adults and they still exhibit egocentric behavior, then the reason could be a cognitive complication.
Those who are egocentric struggle to communicate with others. They do not understand how to read another's body language or emotional state, making it difficult for them to have healthy communication. This also makes it difficult for them to see another person's perspective on an issue and to have understanding of a person's standpoint. Egocentrics only understand their own point of view.
Though an egocentric tends to be perceived as arrogant and prideful, the root of this issue is often due to low self-esteem. With their seemingly self-centered behavior, this person is trying to compensate for feelings of low self-worth and insecurity. This behavior is often exhibited when egocentrics are in a situation where they feel intimidated or inferior. They may perceive another person as threatening aspects of their ego and, therefore, run to an immediate defense of themselves.
Egocentrics develop a superiority complex in order for others to accept them. Egocentrics exaggerate their own abilities and achievements in order to gain recognition. In turn, egocentrics will also mock others, making fun of their lack of abilities in order to be perceived as more talented than others.
You must look at the background of the egocentric to understand where this trait comes from. It often is due to the way egocentrics were raised as children. Often their parents were highly permissive, not giving them enough discipline as children. Egocentrics were often also spoiled or indulged and were given an above average amount of praise, which caused them to see themselves in an unrealistic manner.