Every moment of every day (especially when having quit is still in your addled short term memory) is going to be spent thinking about how much life sucks, and additionally how great it would feel to go back to using again. You can remember how good the first drink feels sliding down your gullet, and how good the first hit feels. Just knowing that you are going to finally get back into your preferred means of escape is both a rush of adrenaline and a relief like knowing that everything is finally going to be okay for a change. Of course, you know logically that relapsing would be a very bad idea. It might kill you. It might kill somebody else who just gets in your way. And as if that were not bad enough, you might end up doing horrible damage to somebody who cares about you, just because they are around you.
Being around an addict is a decision that they make, and you might feel a rush of anger that they would dare to interfere with your life. After all, it is so very easy to lay the blame on another person, since they are making the decision to be around you. You could easily hurt them without even thinking about it, after all. And if you are far enough away from sobriety, you may not even realize what you have done for a long time. And then you can just go back into your altered state, and shrug it off as not that big of a deal.
Of course, you have to choose to relapse, which makes the decision to do so entirely your fault. You might not think in those kind of terms, because doing so might make you feel like a terrible person. Even though your addiction (or that of someone you love) is an illness instead of an evil thing, it is still a choice to be made. And the answer can always be a no. Of course, for many addicts it does not feel that way, but there is always another option.