why do people leave the counseling field?

What a great question! I have a Masters in Social Work (and public health) and worked as a community organizer with homeless people who have mental illness for four years. While not QUITE counseling, I left the field because:

1. I found that I didn't have good work/life boundaries, which meant I couldn't "turn off" work when I was done. As a result, I was thinking about my "clients" all the time and was never able to recharge after intense days and I became burnt out.
2. I left graduate school with significant student loans. The pay in my chosen field was not sufficient to let me begin to pursue the financial goals I had after leaving graduate school.

My advice for people interested in the field is to:

1. Make sure you have good supervision performed by a professional counseling supervisor who can help you process the intense work you are doing so you can (a) understand it, and (b) leave it at work. "Supervision" is a process required in many counseling careers - where you are essentially receiving counseling and management support from someone senior to you - focused almost entirely on you as a professional counselor. It is pretty cool!
2. Develop habits to help you separate work from life. As I've become older, I've been able to do this better. For example, my sweetheart and I try to take a walk when we're done with work. This helps us connect with each other and create a boundary between the work part of our days and the rest of our lives.

From my experience, probably the most common reason mental health practitioners don't continue in the field is "burn-out" caused by the ongoing stress of listening to and trying to respond in a helpful manner to people who are experiencing extremely negative emotional states. It can be difficult to maintain one's "balance" and optimism when people are very depressed, even suicidal, or who suffer from debilitating anxiety or fears. It's difficult sometimes to not feel somehow responsible to be helpful, and taking on that responsibility can have negative effects on the practitioner.