A classroom isn't a factory, churning out the product of educated humans. We can't just go through the motions of something and assume that someone can learn. The process is highly collaborative and highly dependent on getting into the right emotional state. In order to learn, there needs to be trust.
It's easy to understand why children might have a hard time trusting adults. We, as a society (and sometimes teachers and parents in particular) are constantly telling children things that aren't true in an effort to manipulate them. It's "for their own good" sometimes, and sometimes it's "for our own convenience."
Is it any wonder that, after awhile, they become wary?