Steven Levitt asked his readers to guess what Becker thinks is the most additive thing on earth. Here’s how Levitt thinks about addiction:
…an addictive thing has the following characteristics:1) Once you start consuming it, you want to consume more and more of it.
2) Over time you build up a tolerance to it, i.e. you get less enjoyment out of consuming a fixed amount of it.
3) Pursuit of that good leads you to sacrifice everything else in your life to get it, potentially leading you to do ridiculous things to try to get the good.
4) There is a period of withdrawal when you stop consuming the good.
The answer to the question’s here.
Ryu's Blog 
The first thing that comes into my mind is L.O.V.EI believe it\’s the most important thing，or rather the ultimate purpose of human life…
Steven Levitt also think falling in love is the ultimate addiction. I agree that love can be addictive, but whether it\’s the most addictive thing is hard to say. Becker\’s answer "people" is probably closer to my view, but perhaps it\’s just a broader term that emcompass more human emotions and motivations.