More than 50% of doctoral students leave their programs before earning a degree, with some research indicating mental health disorders and stress as the primary reason for the attrition. The culture of doctoral work can prompt paranoia, overwhelming desire to please dissertation committees and other faculty. Multiple studies suggest most graduate students report more stress than they can handle levied by faculty members, and mentorship was drastically needed early and all throughout the program.
http://www.educationdive.com/news/how-colleges-contribute-to-a-50-doctoral-attrition-rate/422189/