I dare suggest that the composer would do himself and his music an immediate and eventual service by total, resolute and voluntary withdrawal from this public world to one of private performance and electronic media. - Milton BabbittI'm not sure I agree with Babbitt, although I think he may not have been speaking of complete isolation in terms of music creation.
Part of my process of learning to compose was in my youth as a musician in the high school band. I heard music that we weren't playing and felt we should be. After some early attempts at brass quintet music, I realized there was more to writing music than just trying to transcribe what I was hearing in my head. It wouldn't appear on the page near fast enough and my imagination/training was not good enough to accurately capture what I thought I heard. The comments from my friends and fellow musicians in the quintet were quite helpful (albeit brutally honest) in guiding my desire to compose. (I chose to ignore the comments advising me to give it up as I'd never amount to anything...)
Some years later I actually did go about getting my degree in music with a focus on composition. Again, during this process the instruction and critique by my tutors and fellow students was immensely helpful. They would hear things in the music I didn't intend and failed to hear things I thought were blatant. This guided me toward a better understanding of what works and what doesn't.
As I moved into my Master's studies, this sort of feedback was critical for my growth process, although I was learning to filter which comments I took to heart and which ones I let fall into the abyss. Still, it was the communication and connection with others that fed my growth as a composer. I could read a great deal from books, study scores for interesting ideas, but it was the comments from musicians and friends that meant the most to developing my sense of style - something I'm still working on.
I've joined the modern age with a website and accounts on Facebook, Google+ and Twitter. The result is a much broader scope of friends who run the full gambit from professionals to enthusiastic amateurs: some are fellow composers, some are musicians, some are just enthusiastic about music. While certainly some of these "new" friends don't comment, some have been extremely helpful. Again, they find things they like or think need work, and point out elements that need clarification.
Babbitt may have reached the point he no longer needed comments and feedback to know where his music was headed. For me, I am thankful for all the great friends I have, people willing to be honest (and yes, sometimes brutal) about my music.
To all those budding composers/musicians/artists of any type: Look to your friends. They will guide you to finding your true-self. They are more than just a touchstone to find what is real, but the support you'll need when it seems all the other's in the world are trying to tear you down.