Adam was but human--this explains it all. He did not want the apple for the apple's sake, he wanted it only because it was forbidden. The mistake was in not forbidding the serpent; then he would have eaten the serpent.
When I read the warning label on the gun that read "Please do not shoot at people," I immediately wanted to start shooting at people. Why did I do something that people shouldn't do? That itself explained how primitive and childish I was at the time.