On the Wagon song meaning
The song describes the effects and interaction of 2 addictions: one is drugs or alcohol, and the other one is love.
The first verse is about the need for intoxication. The narrator says that he has the craving for alcohol/drugs, but doesn't want to slip back into the addiction. And in the second verse love kicks in and everything mixes together. "Today I say sweet things, but tomorrow I'll be making up excuses for my actions"…the narrator is saying that it's not always easy for him to be good enough for the relationship because it's been a long time since he's been in love. And being "on the wagon again" probably stands for being in love.
On the other hand, knowing that the saying "on the wagon" means abstaining from drinking alcohol, one might suggest that the problems in this relationship could be caused by the narrator's withdrawal problems. So, all in all, the song is a description of the difficulties of starting a new life - in love and sober.
There's also a theory that the song is about masturbation, but that's very unlikely.