Meursault is alienated because he does not "play the game," as author Albert Camus once explained it. Unmoved by his mother's death, his girlfriend's love, or the joys or sufferings of other people, Meursault is a man without connections or even the desire for connections. He is amoral, seeing no value in any system which imposes rules and meaning on a universe that Meursault perceives as utterly indifferent to human concerns or beliefs. Therefore, decisions such as helping a man emotionally torment his mistress or killing a man he does not know in broad daylight all mean nothing to Meursault.
Most of Meursault's society does not think in this fashion. Family bonds are viewed as sacred, so not crying at one's mother's funeral is seen as proof of callousness and even immense monstrosity. Most believe that life has meaning, whether that meaning is to make the world a better place or to be as successful as possible in one's chosen field. Meursault will have none of this: he goes about life indifferently, only taking pleasure in sensual delights, such as enjoying good weather or having sex. As a result, he is alienated from a society which demands that individuals order their lives around meaning.