The third reason might have to do with the type of person you are. What I mean by that is are you an urban, suburban, or rural person? The place you serve and live can help answer this question for you. I once took a position in a small rural community and quickly found out this was not for me. I love the big city. I like being close to the theater, good restaurants, and lots of people. Know your personality and look for a church that fits with that. You and the church will benefit from that good match.
Lastly, a reason you may question yourself is a sense of calling. Are you serving in a community that is affirming your calling? When I started out in ministry in the early 90's there was this view that youth ministry was a stepping stone to the next big thing as a pastor, which in most cases meant an associate role, then a senior pastor. I have had a number of people over the course of the years ask me when I am going to move into something more/else as a pastor. What I believe they meant is, when are you going to step up into a more "adult" ministry? I have learned to answer this by using the example of a gifted High School teacher who is in his fifties, still passionate about teaching, relevant and liked by all the students. Is he disqualified because of his age? No, and either should we be. I am really blessed to be serving in a church that asks an entirely different question. They ask, "How can we continue to support you in your calling?" This gives us both the opportunity to explore our options. When I come back and say I still have a passion for students and equipping leaders, this gets affirmed, and we move forward in ministry.
It's really difficult to leave a church. The wonder of what's next and the question of God's call and direction will always come up. If you feel like something is just not right, start asking some questions. Maybe one of these questions will speak to you about why you are serving in the church, and help give you some direction. I definitely believe in longevity in ministry, and would hope our church leaders see it as a benefit as well.