Kristina's mother and most of her family are caught up in there own lives and don't really pay much attention to Kristina, until she gets caught up in all this trouble and starts acting weird. The first time her mom really says that she loves her in the book is when Kristina says to her "fuck you."So she sees that once she starts doing crank and staying out late, her mother starts to show that she cares about her and loves her, so why would she stop if that's the only way to make her mother love her? And she dates Adam because she needs love to make up for staying with her dad who caring about his daughter is one of the last things on his list of things to do. Kristina does crank for the first time because she wants Adam to love her, and he does crank, so her doing crank will impress him and make him love her. She dates and gets high with Brendan because he's beautiful and could easily get other girls but she wants him to just love her and wants to feel close to him. She makes love to Chase because she wants to feel what making love feels like when the person actually loves you (unlike Brendan), and she just wants to feel his love for her.
There's got to better ways to find love, or to get people to really show their love besides hurting yourself for it. One of the things every single human being has in common, is that in some way they are searching for love. Knowing that someone loves you and feeling their love for you is one of the most amazing feeling someone can feel, whether it's your mother, your father, or your boyfriend/girlfriend. Many of the decisions people make every day are made for love. People will do anything to feel loved. And sometimes that anything is destructive, to them or to others. So one of the most difficult things as humans, is finding harm free love and making people love you for who you are, not what you do.