You cannot fulfill the Law by your actions or strength. There is only One who has - and that is Christ Jesus - and He has done so for you.
Therefore, when one preaches "Law" it has to be a law that is beyond fulfilling for the listeners. Otherwise it's not the Law in it's full sternness - it is watered down to manipulation and (potentially naggy) advice giving by the Pastor.
Because if the Law is reduced to something that I can do or accomplish, I no longer need Jesus. This holds true even if some sort of "you do this because of Jesus" line is added.
Consider: Saying, "you need to love your neighbor more" - while true, isn't the full extent of the law. Why? It is utterly possible for me to love my neighbor more - I can look and see a tangible change in my own efforts and actions and thus have accomplished the instruction. Of course, the virtuous pagan can do so as well.
However, in this case, the preaching leads to smug, confident and secure hypocrites, trusting in their works.
This is because the primary theological job of the Law is not to modify behavior. The primary, fullest function of the Law is to reveal the utter depths of sin.
As long as we live, we remain sinners.
Thus, the Law preached in its full sternness cannot be preached in a way so as to provide room for the old false, misleading dream - that by works men effect a change in themselves which moves them beyond sin and gains for themselves heaven.