The Stranger is narrated in the first person by Meursault, who famously announces the death of his mother to the reader in the opening sentence. He then goes on to describe the details of his visit to the retirement home at Marengo and his mother's funeral. At first, the reader might think that Meursault is repressing his emotions, but as the book progresses, there is no indication that he feels any sadness at his mother's death, or that he loved her when she was alive. He describes their lack of communication when they lived together, and his reluctance to visit her in the home.
It seems, therefore, that Meursault expresses no sadness because he feels none, but why he feels none is another matter, which is left open for the reader to decide. A common explanation is that Meursault is naturally "indifferent." He feels no strong emotions about anything, and lives from moment to moment, like an animal. Some critics, and Camus himself, have found an admirable element in Meursault's refusal to be a hypocrite. Some people do not love their mothers, but most find it expedient to pretend that they do, because they know that society expects it. Meursault's lack of feeling, therefore, is more common than it might first appear.