Because basically we are social beings who need others. Like it or not, believe it or not, there is a time in our lives that must have needed someone else help.
Although being selfish cannot harm others, still, as social beings, we need to be empathetic and feel what others feel.
At least we can learn and help each other.
To bring everything back to oneself, to take the biggest part of the cake, to think of oneself before thinking about the well-being of others ... Selfishness is the very opposite of the altruism which aims to always think to make pass the pleasure of the others first. Not taking into account the sensitivity, the timetable or the desires of others, the egoist is deeply convinced that his problems or his projects deserve much more attention than those of his contemporaries. Taking root in childhood, the selfish character is emerging as the baby becomes aware of the total devotion of his parents to meet his every desire and need. Pampered, pampered, fed, cradled, changed ... the baby then forges its future character and if it does not manage to exceed the course of the understanding of its independent belonging to the world, it is possible that as an adult it exacerbates this selfish aspect, constantly believing that his entourage is entirely devoted to him.
But selfishness becomes an exasperating defect in pushing to the extreme, selfishness can become a real handicap in the relationship to others. Whether in love, friendship or professionally, it is difficult for those around to endure the individualistic side of the selfish. In order to always succeed, to rise socially, to maintain control over one's relationships or to value oneself in the eyes of the world, the egoist can elbow and manipulate his surroundings so that situations always turn to his advantage. Once we realize his ride, difficult to trust him. Always seeking to satisfy his desires to the detriment of others' feelings, the egoist systematically distorts his relationships with others, which often pushes him to isolation or even depression. Contrary to appearances, the egoist is sorely lacking in self-confidence and, by extension, trust in others. To "heal" of this defect and to find healthy relationships with others, the first step is to become aware of one's selfishness and to appreciate the implications this defect generates in one's life.
This is totally not true, being selfish is harmful depending on the magnitude of the situations you must know that selfishness means putting oneself first before considering anyone else in any situation, for example someone who is selfish will save themselves first whenever there is a fire outbreak and even when they're definitely definitely free from the fire, the willingness to go back and save a person who is still inside the fire will never be the priority of a selfish person, in this situation that person's selfishness has harmed a person, because allowing that person die in a fire means it's harmful and may end up leading to someone's death if another unselfish person do not come in to save them.
Selfish is an anti social value that's makes a person place themselves on top their scale of preference, it means they have to willingness or urge to put people's need before their needs and as a result of this people who are around him may never enjoy the virtue of bravery from him, in desperate times he may consider himself the only person deserved of attention which may cause either endangerment to people that a selfish person could've easily helped, so being selfish can be harmful.
This is by no means whatsoever, veritable, being immature is dangerous depending upon the tremendousness of the conditions you ought to appreciate that narcissism suggests putting oneself first before considering some other individual in any situation, for example someone who is vainglorious will save themselves first at whatever point there is a fire dispatch and paying little regard to when they're clearly absolutely free from the fire, the status to return and extra a man who is still inside the fire will never be the need of an uneven individual, in this condition person's pubescence has harmed a man, in light of the way that allowing that individual pass on in a fire translates it's stinging and may end up actuating someone's whipping if another unselfish individual don't come in to save them. Energetic is an enemy of social regard that is makes a man put themselves on top their size of tendency, it assembles they have to motivation behind constrainment or need to put people's need before their necessities and in light of this people who are around him may never welcome the wonder of power from him, in real events he may consider himself to be the principal individual maintained of figured which may cause either hazard to people that a narcissistic individual could've easily helped, so being radical can be perilous.