I've been reading some interesting news on the web, most of it tied to education, but the same holds true for writers. It turns out that too much self-esteem is not good. That's understated. It's downright dangerous.
The idea is that excessive positive self-esteem is constant reinforcement that a person is, well, perfect, and can do no wrong. When confronted with obvious failings and shortcomings, the reinforced behavior of the person's ego is unable to reconcile, and so places blame on external forces instead of recognizing imperfection.
In short, students who have too much self-esteem will resort to blaming teachers, work, assignments, or other external forces rather than embrace the idea that they are the ones who have failed. This excessive self-esteem prevents people from taking responsibility and wanting to improve themselves. After all, they've been told repeatedly that they are perfect, so there is nothing to improve. They have a locked mindset that prevents growth and change.
Writers are not immune from this, either. I believe writers, especially fledgling writers, receive too much positive feedback on their writing from friends and family instead of other writers and professionals.
Professional writers (and teachers for students) are under no such illusions about assessing abilities. It puts everyone in an awkward position, and can result in harsh feelings.
It's no wonder that many agents have embraced not responding to queries.
Teachers do not have that luxury.