Well, I said most existing humans are opposed to wireheading, not all. ^_^;

Addiction might occur because: (a) some people suffer from the bug described above; (b) some people's utility function is naturally "I want to be happy", as in, "I want to feel the endorphin rush associated with happiness, and I do not care what causes it", so wireheading does look good to their current utility function; or (c) some people underestimate an addictive drug's ability to alter their thinking.