Every person needs someone who he can idolize and follow so that he gauges his own talents, achievements, and progress in comparison to that person and constantly improves. This is exactly what self-confident people do. While interacting with others, reading about others, and putting their ideas in public to gain perspective, they constantly improve their skills and knowledge.
But what happens when people stop doing that? They tend to lose focus, as they don't have any anchor left in their life who can stabilize them when they are getting adrift. When interaction with society is stopped, a person tends to think about himself and that reflects in the way he accepts criticism. Even constructive criticism for his work will appear personal criticism for him, as his approach would have become extremely self-centered by then.
People with low self-confidence tend to think they have no talent or skill to complete a given task and be appreciated. In fact, genuine praise for a good effort comes to them as a surprise and they perceive that as fake appreciation.
People need some positive image about themselves so that they can identify their strengths and weaknesses. People with low confidence often lapse into a mental state where everything becomes a weakness and every observation appears personal. This leads to further self-degradation.