Ah, the Ego.
The part of the psyche that defines you as an individual, and considers itself (read: you) a separate entity from the rest of nature and even the entire cosmos, the ego encourages people to value themselves as well as their personal wants and needs instead of just being an Extreme Doormat. While necessary in moderation, allowing one's ego to become overinflated leads to self-centered and downright embarrassing behavior. Such an excess of self-importance can be called egotism.
The egotist seems to think that the world ought to revolve around them and their every little whim, and is ready to walk all over other people in order to get what they want. They expect others to wait upon them hand and foot, make way for them on the sidewalk or let them cut ahead in line, help them first even if there are other people in greater need, and sacrifice their own best interests for whatever the egotist demands. They don't care about how their actions might harm other people, yet display Moral Myopia when someone else does something to harm or inconvenience them. If they do understand that the world doesn't revolve around them, they'll see this as a bad thing in need of rectifying.
Exactly why they think they're important and everyone else isn't will vary from case to case. For some, it could be the belief that they're superior to normal people because they are more beautiful, talented, favored by a deity, etc.—which may or may not be true—while others are simply so lacking in empathy that they can't imagine anyone else's pain or feelings being as real as theirs. On the highest end of the ego scale is a terrifying form of Obliviously Evil, when the egotist is unable to comprehend that they're doing anything immoral because they are unable to comprehend that other people have feelings at allnote .
Selfishness is a big part of the definition, but even beyond that an egotist thinks of themself as being the focus of everything that anyone else does. They assume that everyone knows who they are and what they can do, which might be plausible if they are actually important, wealthy, or famous, but becomes Small Name, Big Ego if they are actually much less special than they think. If anybody fails to recognize them or treat them like royalty, they feel intentionally snubbed or insulted. What’s more, if they end up as collateral damage of someone else's scheme or even just an honest mistake, they interpret it as a conspiracy to harm or humiliate them specifically. This is despite their own tendency to forget the names and faces of "little people" who they consider beneath them, and to say But for Me, It Was Tuesday when they can’t remember a person they mistreated.
It's hard or impossible for them to imagine that other people have their own lives outside of interacting with them. Not only must their friends, family, and adoring public be thinking about them all the time, but their rivals or enemies are surely out there obsessing about how to bring them down, no doubt because they’re jealous of the egoist's style and success. An egoist who’s an Attention Whore or a Card-Carrying Villain might go so far as to think there’s No Such Thing as Bad Publicity, since the only thing worse than being talked about negatively is not being talked about at all. An egotist with delusions of grandeur may have a one-sided rivalry or hatred for a different character who hardly even notices them. If normal criticisms fail to work on an egoist because they lack any sense of morality or shame, the most effective tactics for deliberately getting a rise out of them are mocking how pathetic and ridiculous they are, calling them "boring", or giving them a "The Reason You Suck" Speech about how they're delusional to think they’re so important.
A healthy ego exults the strengths of an individual in an overt way, which—on balance—can benefit everyone. An unchecked ego, however, is mainly preoccupied with blocking out "unacceptable" thoughts that are hurtful or belittling to itself, even tuning out the entire world if necessary. Toddlers have this attitude by default, as their brains are not developed enough to understand that other people have different viewpoints. We're less forgiving of (non-disabled) adults who behave this way, also termed assholes.
When the cause of the attitude is being raised in a family of great power and wealth, you have a case of the subtrope Royal Brat. For religious cases, see Egocentrically Religious. Overlaps with Moral Myopia when debating the rights of others versus the rights of oneself. Also overlaps with Opinion Myopia, where the egotist thinks any ideas or beliefs they personally disagree with are wrong. An egomaniac who likes being in the limelight will usually be an Attention Whore. When combined with a massive sense of self-preservation, we get the Dirty Coward. Muse Abuse is often caused by an artist believing that they're a genius whose talent puts them above conventional morality, and thus they have a right to torment people close to them for inspiration as long as it makes their work better.
See Narcissistic Personality Disorder and Anti-Social Personality Disorder for when it veers into villainous territory.
