Now you know the WHY

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Step 1
Note the fact that it's their way or the highway. They equate compromise with loss. Egotistical people also feel that they have to be the center of attention to validate themselves. They often neglect the needs of those around them and think only in terms of what will suit them.
Step2
Learn that a person with a big ego needs constant praise. This happens because they're shielding low self-esteem with a capacity for achieving goals and entering contests. Unlike a person with high self-esteem, someone with a big ego needs the acclaim of others to boost their secretly underdeveloped self-image.
Step3
Observe that someone with a big ego drives others away. The wear and tear of dealing with someone who's demanding and self-absorbed causes co-workers and associates to avoid the egotistical party. Catering to a big ego becomes a drain and people eventually cut off contact with them.
Step4
Realize that an egotistical person often believes that another's success means they've failed. They feel better when people around them achieve and earn less.
Step5
Understand that egotistical folks always compare themselves to others. They like to point out that they've made more money than their neighbor, or they become inflamed and try to outdo someone who has a bigger car or fancier clothes by making loftier purchases. They like to "keep up with the Joneses."
Step6
Recognize that someone with a big ego likes to "undermine" others. Someone with a big ego wants to make sure that other people don't get ahead of them in business or life in general, so they're constantly on the defensive.

That is ripped from http://www.ehow.com/how_2156763_tell-so ... g-ego.html.