Most writers, including me, have self-confidence issues, or perhaps it is artists in general. I believe it has a lot do with putting ones work out for public criticism and human nature to take that criticism to a very personal level. If someone says our work is stupid, we take that to mean we are stupid. This thought pattern mean it is difficult to put ourselves and our work out there. As maturity comes a small willingness to show someone else that work, but their criticism can be the difference between hiding any sign of being a writer or letting the world see it.
As self-confidence builds it comes easier to seek out criticism and use it to our advantage. Having a trustable person read and provide feedback is necessary to putting out the best work. This person has to give you feedback that is useful without destroying any self-confidence you have built up. And if you are a person who has been trusted to give criticism, do not just tell them their work is stupid. Tell them their character is shallow and unsympathetic, which makes it hard to believe the character solves the problem set out in the story. It help build up the person’s self-confidence, which will help them be willing to share more of their work.