Writing is a solitary activity.
I retreat to a quiet place, put on my music, don my earphones, find that "door" in my mind, open it and let the story out.
However, the product of writing, for most writers, is intended to be read and enjoyed by others. That's the reason the real work in writing revolves around editing, rewriting, cutting, adding and tweaking to make the story appealing and enjoyable.
When I write, I have an emotional investment in my story, in my characters, in my words. As one respected member of my writer's group says, we writers love our words and are loathe to cut any from a WIP (sometimes, I find that it's the word or phrase I'm most in love with that most needs cutting).
That's why critiques from other writers are so valuable. Other writers don't have the emotional investment in my work that I do. They can see if it's interesting, moving, or if it slogs along too slowly. They can pinpoint areas that need work, that need clarification -- the point I may have intended to get across (and thought I did get across) may not be apparent to others. Critiquers can let me know that my point is missing-in-action, that it needs to be made in another way so that the reader gets it.
Good critiques are a valuable, sometimes painful, sometimes affirming, part of producing the best story I can write. And will help give me the best chance that my novels will be enjoyable to read and will, therefore, be read.