As a private detective (emphasis on the word "private"), Marlowe leads a very lonely life. He keeps his own company and can usually be found whiling away the hours at the bottom of a bottle. Sure, we can chalk up Marlowe's alone time to the fact that he's chosen a particularly solitary line of work, but we can't help but wonder if it isn't a symptom of Chandler's America as a whole. 1930s Los Angeles with its murderers and blackmailers is primarily a city of distrust where each person has to look out for his or her own selfish interests, before finally succumbing to The Big Sleep.
Marlowe chose the loneliest job in the universe because he was a lonely guy to begin with.
Marlowe's solitary lifestyle is a way of distancing himself from the corruption he sees every day.