By definition, constructive criticism is justified criticism. If the reader tells you why they didn’t like something they are doing you a favor. In fact they are doing you a far greater favor than a thousand people saying your piece is ‘good’.
Think of how children react to a movie they love and inevitably watch two hundred times. They quote lines from it, they sing songs from it, and they mimic memorable actions. Children are also fairly vocal about their dislikes regarding a movie. If a section scares them or grosses them you, you will know about it. To be an effective critic you need to revert to being childish, with an adult thoroughness.
Now here’s the rub. The critic doesn’t aim to insult. They are being hard on the piece of literature in front of them because it disappointed them. They expected something better and they got something worse. If you reverse the logic for a moment, the more relevant nasty things they say points to the fact the critic seriously considered your work.
Some people get upset when they hear extensive criticism on their first paragraph(s) and immediately snap into ‘you didn’t give it a chance’. If this happens with multiple people, then it is fair to assume your beginning needs work. A strong beginning is the store front that gets people to come in and buy when they’ve opened the door and taken a glimpse. So, first things first, you need to come to expect this sort of critique early on.
The number one thing that buries new writers is the assumption that everyone will love the characters as much as they do. You need to earn this love. If you write your first few words under this pretense you will be telling an inside joke to people that don’t know the context. Lesson one of getting thicker skin? Accepting you start with nothing.
The correct answer to anyone who takes time out of their busy life to give insight: Thank you.