Quote 1
"The Madman Underground is all about how much everybody needs each other, and hauling my ass out of here is all about not needing anybody. And the Madman Underground is all about telling your story to people who already heard it and like you anyway, and I want to live someplace where I can have my story, not just some things that happened to me." (20.42)
Karl fortunately comes to the realization that he's never going to be normal, and since that's the case, he might as well keep his friends and the unique yet bizarre situation they have.
Quote 2
"You said you will have one day," I said, calculating, "but you met my mother on Thursday night—"
"And then got so chickenshit-scared that I got drunk after work on Friday and didn't wake up till ten A.M. Saturday. Scared she'd turn me down, scared I'd get there and she would have forgotten, mostly just scared. If there was ever a good reason to stop drinking it's having done something that stupid." (22.58-59)
Bill is a good example of what alcohol does to people—it makes you scared of facing life, which just makes you drink more. Little does Bill know that his decision to get drunk rather than go to Put-in-Bay with Beth actually causes her to get drunk as well.
Quote 3
"Coach," I said, "my parents were screwed-up people who drank together a lot […] Mom and Dad had drunk fights and drunk make-ups and drunk sex, and I was scared to death a lot of the time. They tucked me in when they were drunk, and I got myself cereal while they sat at the breakfast table holding their heads and groaning about their hangovers. They loved me and they fought each other and they did stupid things." (26.102)
So, what does all this boil down to? At the end of the day, alcohol has been a part of Karl's life since he was a little kid. Ugh, right? In this way, alcohol is the cause of all the problems Karl faces—it probably played a role in killing his dad, it's wrecked his mom, and ultimately, both of them tried to take Karl down with them, too.