I always used to wonder why pastors leave. If you have attended church for any amount of time, you will probably have experienced this event at least once. There was a stretch, when I was younger, that we had 4 different pastors in a 5 year period. It's also possible that you have never experienced a pastor resigning and may be asking why.
This is by no means an exhaustive list, but here are 8 reasons I have encountered myself or seen in others:
1. Burn-out
Although you may feel that a pastor has an easy job, there is much more to it than meets the eye. Preaching, teaching, counselling, praying, fasting, visiting, leading, training, and organizing can take a tremendous amount of time and effort. If a pastor doesn't have a strong ministry team and/or volunteer team then he may find himself in a state of burn-out. Burn-out is when you just hit that brick wall where you have nothing left to give and you realize you have been leading on empty. The book below describes this in great detail from a pastor's perspective.
2. Discouragement
Pastors take on new works or churches with the goal of succeeding in winning people to Jesus. Any pastor wants to win souls. That goes without saying, but everybody has a different tolerance for timelines for that growth. One man may be working on a 10 year plan while another may expect immediate results. Either way, once they reach the end of that timeline, or even before, if the results aren't what they expect, discouragement sets in.
Another type of discouragement is not number related. Pastors invest time training, counselling, and working with people to grow their walk with God or their ministry. Sometimes many years are invested and if somebody decides to give up or turns their back on the pastor, it can be very demoralizing.