Jesus tells an interesting parable in Luke 17: 7-10 as he sets his face towards Jerusalem. It is a principle of servanthood that applies to the person who would seek to serve in God's kingdom.
To paraphrase it: "Does the boss see his hardworking crew sweating at their labor, take pity on them, and have them sit down to rest their weary bodies while he cooks and serves them? No. Instead, the servants must continue to serve their boss, even when they're tired. The boss doesn't thank them for doing their job because, well, it's their job. Instead, they merely say to themselves, 'Hey, we're laborers and we're just doing what we were hired to do. No biggie!"
Don't get me wrong. I believe in saying "Thank you" to someone who has done a good job in ministry. Encouragement is, after all, one of the gifts (see Rom. 12: 8).
But for the serious Christian, there is something obligatory about ministry. This is part of what Jesus is getting at here. When I am tired and worn out, and people seem to take my service for granted--so what? I'm just doing what I'm supposed to do.
It is why Paul seemed to love to refer to himself as a "slave of Christ." Normally, a person in Paul's day would not boast about being a slave. He saw himself as not just any slave, but a slave "of Christ!"
Thank you, sons of Korah, for some great lyrics: "I would rather be a doorkeeper in the house of my God than dwell in the tents of the wicked" (Ps. 84: 10).
Perhaps we can begin to adopt the same kind of attitude. When ministry seems difficult and people ungrateful, instead of getting upset, we could just say to ourselves, "Oh well. No biggie. I'm just doing my job."