Now, this fear aspect has a bad name among many modern Christians. Whenever I bring it up, people will dispute with me as to whether this is really a good thing. Shouldn't we tell people that they should only do God's will out of sincerity? I hear this a lot and so here's a couple of thoughts on the question:
First, sincerity tends to be more of a modern concern than a pre-modern one. It is very culturally specific to us. I personally think it's overrated. We are obsessed with the inner life of humans and therefore we do not consider anything to be authentic that is not directly from the heart. Christians can't take this position for a number of reasons. For one thing, Jesus tells us that heart is the seat of all wickedness and therefore just because we have a sincere feeling, it does mean it's a good feeling. In fact, in light of original sin, it pretty much never is. This has had horrible consequences for our society, not least in the area of divorce. People don't feel the spark in their marriage any more and so in order to be sincere and authentic, they end up putting their kids through hell. One could name any number of social maladies that have arise from this non-sense, but you get my point.
Secondly, I think that the recognition that we will not always do good things for the right reason is liberating. People forget that Luther's problem was not merely how he could find a gracious God, but how his obedience to God could be pure. What he concluded was that he could only love God if God took away the demand for his love. Nevertheless, sin still clings to our flesh until we die and therefore we will not always feel this gratitude and desire to do God's will perfectly in this life. We should therefore look at God's commands and force ourselves to do God's will, trusting that in spite of our sinful attitude we are still loved by God. This imputation of Christ's perfect righteousness frees us from the burden of having to be sincere.
What most people don't realize therefore, is how incredibly liberating this is. Luther believed that he had to every work in the monastery with sincerity, otherwise it was no good. But once one has the imputation of the righteousness of Christ, one can be honest with one's self about the falsehood of one's own heart. One need not torture themselves about the fact that they are only doing certain duties out of compulsion. In my own life, I do many things which I don't particularly like, but I know are God's will. How much more odious they would be if I had to deal with the endlessly struggle of having to make myself really want to do them! Therefore, although our hearts are in conflict with God's law, we are loved and accepted in spite of it for the sake of Christ.