The word pastor means shepherd and the word shepherd is a metaphor. It is used to describe leaders of the church because it better summarizes their responsibilities than any other term. It is good to step back and think about exactly why God describes the church as a flock of sheep and Himself as the chief shepherd, the way in which God has committed these sheep unto Christ and Christ’s own great sacrifice on their behalf, laying down his life for their good and what God’s calling them as under-shepherds of His sheep really means. But, certainly one thing this description implies is that a pastor will love, care for, and watch out for the spiritual good of his people, going before them, seeking to protect them, feeding them, defending them, as servants of the chief Shepherd. The title shepherd is really a honor, but it involves sacrifice. It is a description of a servant not a king. Therefore, it’s not surprising that this term became less and less used in the church and titles like bishop or guide or president or even reverend came more and more into use in its place, as these titles imply more glory and authority than that of a humble shepherd of sheep. But this is the name God uses to describe pastors and their work is to feed God’s flock.
John Owen in Simple English, The Nature of a Gospel Church