It's also helpful to have another pair of ears. Voices are inextricably interwoven with who we are. It's almost impossible not to be blind to what needs help and/or be overly judgmental and harsh with ourselves. A voice teacher can help keep things in perspective, gently helping you to appreciate your gifts or giving you a kick in the butt when you need it.
Most importantly, singing is about communication. We spend so much time texting, tweeting, Facebooking and emailing, but, singing goes deeper. It speaks to the soul. It is a much richer experience when you open up your heart to sing to have someone standing beside you as witness, cheering you on and saying, "Yes, I hear you."