I had no pressing business in New York, but I made it a point to go, and I presume it was for the same reason I'd helped old Jesse frame a new identity for himself and then got him a job in Boston—because I was now addicted to entering the world of Jack Diamond as fully as possible.
Marcus states this right before he decides to go visit Kiki in Manhattan. Marcus, Kiki, and Alice all go through periods during which they consider ending their relationships with Jack. Whether they are fed up with Jack's unsavory mobster tactics, his penchant for sleeping around, or the fact that he is always leaving them alone with one of his henchmen, each character occasionally decides that the relationship is not worth it. Alice and Kiki do actually leave, and at one point Marcus considers himself no longer in Jack's employ. But they all come back. In a sense, they are all "addicted" to Jack, as Marcus says in this passage. Marcus is looking for something more interesting than the life afforded a small town lawyer; Kiki realizes that being Jack's broad will make her more famous and sexy than her dancing ability ever could; Alice is challenged by her Catholic faith and her wedding vows to keep wandering Jack by her side. But, more basically, they are all attracted by their association with Jack's fast- paced, bad boy, dangerous lifestyle.