The moral of this story is "don't do drugs, kids." The pot-smoking young hitchhiker ends up dead a couple pages later. We doubt she would have survived if she had not asked for marijuana, but McCarthy includes this dialogue for a reason. Drugs are bad news.

Llewelyn is talking to the unnamed hitchhiker here, and he's basically telling her that it's a bad idea to stick around with him. (He's right.) However, he can't make that choice for her; she has to decide herself. And she chooses wrong.