Meursault's lack of emotion has often confounded readers of The Stranger. He is more bored and uncomfortable at his mother's funeral than he is aggrieved. He does not love his girlfriend, nor does he have any real friends. He seems perplexed when other people are bothered by his lack of explanation regarding why he killed the Arab. It would be easy to simply say that Meursault is a sociopath and leave it at that, but Camus is probing deeper questions by presenting Meursault as such a detached person.
Meursault is an embodiment of certain aspects of absurdist philosophy, which states that life has no inherent meaning. Events happen. People behave in ways that are often irrational. Meursault regards events such as his mother's death or Marie's marriage proposal with the same indifference, since he endows neither death nor marriage with the same value that traditional society does. For him, all of it is meaningless. He even tells Marie that while he would marry her if it would "give her pleasure," he does not love her and prescribes no greater value to their relationship in marriage than he does outside of that bond. Hence, Meursault's lack of emotion represents an unwillingness to graft meaning or morality onto an indifferent universe.