In I Peter chapter 5 we see Jesus referred to as the "Chief Shepherd," which amounts to the same thing as "Senior Pastor." Christ is the only one. Therefore, how does anyone else dare to refer to himself as "Senior Pastor," "Lead Pastor," "Head Pastor," etc.?
In reality no mere man can actually take Jesus' spot as Senior Pastor of His church. However, anyone who takes on that title confuses the situation for all within that local body. The people then tend to look to that man for leadership instead of looking straight to Jesus.
Of course not all pastors take the title "Senior." For this I am thankful. Despite this, enough of them do that it causes all sorts of problems for the church.
(This post is part of a series entitled 25 Reasons Professional Pastors Should Resign.)