Perhaps we simply spend too much time alone in an imaginary world, populated by characters of our own invention and that leads to a skewed perspective. Haig puts it down to the author needing to feel things intensely in order to convey an emotional truth through writing, which is probably true too. Writing fiction can be like undressing in public. It's personal and intimate – that's what makes readers engage, so when someone doesn't like your book it feels like a rejection of yourself rather than the work. It may be a generalisation but novelists as a breed are probably more introverted by nature (it's the temperament that makes it easier to be alone long enough to write whole novels) and consequently more thin-skinned than many. But I don't think it is just the writer who is sensitive to damning criticism, I think it's everybody. It's just that in the real world people who hardly know you don't tell you what they really think. If they did society would go completely awry.
Reviews however, are an important platform for the truth and by publishing something we are inviting opinion, indeed opinion is an essential part of the process. Readers need to know what other people honestly think before they buy, and book buyers aren't blind to the fact that such opinions are subjective – to buy a novel and not enjoy it is not the same thing as a buying a faulty toaster. I find it helps to look at the reviews of really successful authors – yes, they get single stars too – and remind myself that no one is everyone's cup of tea.