In the novel The Stranger Meursault is an existentialist. What seems to be the only thing motivating Meursault in his daily life? How does this affect his relationship with other characters in the novel?
The only thing to which the main character, Meursault, responds in the text are his physical demands - hunger, thirst, sex drive, etc. For example, in the first chapter, Meursault doesn't respond emotionally at all to his mother's funeral and death; instead, he mentions many times how hot it is and how much the sounds of the old people crying annoy him. He spends his time throughout the novel in purely sensory experiences, one of the main ones being cigarette smoking.
Later, in his relationship with Marie, Meursault is driven by his sexual lust for her. She asks if he would like to get married and he says he will but he's not sure if he loves her or not, and probably doesn't. He has no emotional connection to her and only misses her in prison because he misses having sex.
Finally, one of the most important sensory responses is the one in which he kills the Arab man on the beach. Following a tense altercation, Meursault goes down to the beach. He is hot and uncomfortable, and then the Arab holds up a knife that flashes light in Meursault's eyes. This annoyance sends him over the edge and he shoots and kills the Arab with no provocation. This is the event that sends him to prison and for which he is eventually executed.
This trait affects his relationships with characters in the novel in that they are all surface - his relationship with his mother, Marie, even the Arab whom he kills - are not full of excessive emotion, or really any emotion at all. His interactions are all in the moment, without regard to the future or the past.