I came across a post on Ed Stetzer's blog,
In your estimation, what are the top 3 reasons Christians don't share the gospel with the lost?
Aside from general disobedience, I believe most people are apathetic about evangelism because they do not understand the implications of the gospel on their lives. They fail to understand who they are in Christ and what it means to live in obedience to Him. Second, people are afraid of what others will say or think about them. The fear of man is their primarily concern. Finally, some people think that they do not know what to say to others. They fear being asked a question that they cannot answer. Or, they fear giving someone incorrect information. Since they do not see themselves as "evangelists," they decide to leave such Kingdom labors to the "professionals."