Why do people involved in children's ministry sometimes struggle?

I've just come back from visiting some very good friends from my long gone teenage years and they were describing their church's emergence (good news!) from a period of meltdown amongst those who previously led their work with children and families.

Working with children and teenagers for a church, as either a volunteer of a staff member, is a very challenging position and you need to be totally, totally focused on God's call to do so. This doesn't mean that if you suffer stress or hard times whilst caring for children, dealing with parents and relating to lots of volunteers that you are necessarily an in a bad place emotionally (though you could be!) as I think the following bravely articulated article outlines so well.
As George Barna said (not me!) children's ministry is the one area of ministry where best attention should be given at the highest level of church leadership.....he wrote that as a researcher and not as a children's minister of any kind....If this is in any way true, then it's probably going to be a target! Another children's pastor recently asked the question: think of the three areas of your church life you'd like to knock out if you were God's enemy. I'd say: work with children, work with teenagers and worship.

There hasn't been comparable research done on this for children's workers and pastors, but at college I was told that the average turnover time for youth workers/pastors was a mere EIGHTEEN months. This does seem to be substantiated by this post and this one.

In my previous church there have been three people employed (one after the other) to work with teenagers in a seven or eight year period so this statistic on first glance fits, if you were a young person throughout this time - although in reality each stayed longer than 18 months and the 7 or 8 years is the span inbetween. What has been your church's experience of this? How can this be reversed? Should it be or is turnover good?