I think it's in the movie Rocky III when Rocky's old trainer tells him that what has happened to him is "the worst thing that can happen to a fighter: you got civilized." The same thing happens to pastors too. Every year, thousands of bright eyed young men graduate from evangelical seminaries full of starry idealism about how they are going to preach the Word, reach the lost, and equip the saints to be a city on a hill. Within six months, many of these will be discouraged and depressed. Within three years, many will be out of the ministry. Among those that survive the initial onslaught, many will survive not as dreamers, but as domesticated drones, finally content to serve as a community chaplain and a poorly paid (usually) lecturer about the great God of the Bible.
What makes the difference? It isn't training, for all of them get good schooling. It isn't spiritual discipline or theological fidelity, for there are many devoted disciples who become frustrated pastors. It isn't giftedness, for the landscape is littered with gifted men who left the ministry. It isn't even personal holiness, for I know many holy pastors who aren't in ministry full-time anymore.
What it is, I think, is whether or not God's Spirit is moving in a particular church. Which means whether or not God's Spirit is moving in particular people. But as Jesus said, "the wind blows where it wishes..." The reason Tyndale, Hus, and Wycliffe died reviled and cursed rather than honored as a great reformer like Luther or Calvin is that they were spitting into the wind, like many pastors do today. The challenge is to remain faithful in your ministry until the wind changes.