There is a very interesting discussion going on here at the Reformation 21 blog on why it is so much easier for preachers to motivate people to holiness by railing at their sin than by pointing to the beauties of God's grace. I have a different take on this, I think, although Carl Trueman's observations come close to the mark.
Like Carl, I do think it has to do with ministers and maturity--but it is not chronological maturity nor general life experience. I think the real issue is personal spiritual maturity, or to change it, emotional and spiritual health. [Here, in particular, I am channeling the first principle from Peter Scazerro's The Emotionally Healthy Church.] If ministers are unable to look deeply below the surface of their own lives and deal with themselves graciously, it is going to be difficult for them to point others to God's grace.
What is more common, in my experience and in the lives of those I know well, is this: typically, those who become senior pastors are fairly responsible and driven people--in order to go through the Presbyterian educational and ordination process, one needs to be! The way they are able to function in ministry effectively (and deal with the stresses and challenges of ministry) is to become very disciplined and responsible people. The way this happens, in turn, is to hold oneself to a standard, whether a checklist or some other device.
Now, what ends up happening is when that checklist is not met, or when the minister leaves the office, if he felt like "I didn't get anything accomplished," guilt creeps in. And most of us deal with that guilt and/or shame by saying, "I'll do better tomorrow: I'll run faster, work harder, do better." We hold ourselves to an extremely high standard (perfection?) and feel badly when we are unable to accomplish that standard. In other words, we deal with ourselves by means of the law.
This pattern of law--standard, failure, guilt, personal recrimination, and repurposing--becomes the pattern for our spiritual lives as well. We struggle with some personal sin--perhaps losing our temper at home--and we set some standard (e.g. in this argument, I'm going to lower my voice and not yell). However, the wife knows how to push all our buttons and we fail. Guilt and shame arise as well as personal recrimination; we rail at ourselves for our failures and we purpose to "do better next time" (i.e. a new standard).
It is no surprise, then, when we take this pattern into the pulpit and preach law (standard, perfection) to our people. I've seen long-time ministers do this and as well as newbie ministers (hence, disagreeing with Carl's basic premise). The "solution" to this is to turn from the law to Gospel: to learn new, gracious patterns of dealing with ourselves spiritually, which will in turn lead to new patterns of dealing with our people ministerially. To put it differently, the solution is to root ourselves, our own spiritual lives, in the Gospel of God's passionate love and abudant grace.
For me, one of the breakthrough, aha moments was reading Bryan Chapell's Holiness by Grace a couple of years ago. Learning to rethink my own patterns in the light of what is true about me (I am united to Christ, right with God, holy in his Son, a beloved child, and glorified already) has given me courage to deal with the remaining corruption in my heart as well as produced a spiritual compassion for the weakness and sins of others that I hope comes out in my preaching and pastoral care. Still, learning new patterns is difficult--I take heart from the fact that Rose Marie Miller's journey in learning to deal graciously with herself took ten years.
In the end, I continue to remember that it is by God's grace that I stand (not by my continuing commitment to the law); this alone can give me confidence and assurance for life and ministry and, ultimately, death (cf. Romans 5:1-5).