Churches can unconsciously move from a biblical agenda of proclaiming the Gospel and reaching the lost to an organizational agenda of pleasing people and keeping everyone happy. Everything leadership does should equip the saints to reach the lost, not to please the crowd to prevent the loss. When Jesus was confronted about people leaving because what He was saying bothered some, He didn't try to change, but He gave others the opportunity to leave if they wanted.
God is responsible to keep people. As a matter of fact, He said that He will not lose anyone. Why do people leave? First, some people have an agenda that is not the agenda God has laid on the leadership of the church. These may be good people that God will use elsewhere. The incident between Barnabas, John Mark and Paul is an illustration of good people going in different directions because of different agendas.
Some people leave because they don't know God, even though they profess to know Him. These are folks who want God on their terms and will continually run from the truth. Either way, trying to keep people with different agendas or lost religious people with personal agendas is not healthy for the church (and especially leaders).
We are called to reach people with the Gospel and to train up others to reach more. God is responsible to keep those who have turned to Him. The departure of people is never fun but it is a reality of ministry. If Christ's ministry had a back door, why do we think that our ministry will not. When people leave it may be a time to reflect and inspect, but it is not a call to change agendas.
Church leaders fail to be fishers of men when they become keepers of the aquarium. While we are spending our energy pleasing the 90, we fail to reach the 9,000 at our doorstep who have not heard the Gospel. A great awakening will come when we become "reachers" and stop trying to be "keepers".