Too often we think that God asks people to do something--teach a class, preach a sermon, give generously to support the work of the church, go overseas, serve the needy--and we lose sight of just exactly who is asking. To paraphrase A. W. Tozer, we may view God as coming to us hat in hand asking for assistance. We understand that God wants to us to do good works, but we forget that He asks us to give what He has already prepared beforehand.
Jesus told the Samaritan woman "Give me a drink." Her response shows that although she certainly thought it an odd request, she never doubted for a moment her ability to give Jesus what He asked. She missed the point of the request. The Creator and Lord of the universe did not need to sit at a deserted well and wait for this woman to provide what He was lacking. The point was for this woman to realize what she was lacking.
Even as believers, we can easily accept the idea that we're saved by grace through no merit of our own yet act like God needs us to give Him a drink of water. We never doubt for a moment our ability to serve God (until He graciously allows us to reap the consequences of working in our own strength). He doesn't ask us to serve because we have something to offer Him. He asks because He has something to offer us.
Next time you believe God is directing you toward some ministry, stop and remember who it is that is asking for your service.