Some people have a really hard time saying "thank you".
It's not that they are ungrateful or just jerks. In fact, some of the people I love and respect the most have trouble with saying "Thank you". I know they appreciate what I do for them or what God has blessed them with or whatever, but they have the hardest time looking you in the eye and giving a heartfelt expression of thanks. I realized last night, though, that saying "Thank you", whether to God or to another person, is an act of submission and that is why it is so hard for some of us to do. Saying "Thank you" ultimately puts others above us and by our openly admitting that, it makes us vulnerable.
By giving a truly heartfelt "thank you", you are telling the person you received some sort of positive benefit from what they've done. To do that, you have to admit either you needed them to do it or your life was made better than it would have been otherwise had they not acted. Being appreciative requires us to admit weakness. It may not be huge weakness, but it requires a person to admit they are not capable of doing everything by themselves.
To go one step further and actually express thanks to another then requires submission. You don't just acknowledge to yourself that you needed them, but you make the effort to verbalize between the two of you that you feel like, because of their effort, you are able to put them above you and say "You were able to do something for me that made a positive impact on my life/project/etc".
We as Christians don't usually have too much problem giving thanks to God because we focus so much on it. Unless we are really mad at God or are trapped in a cycle of self-pity, it is easy for us to heap tons of thanks onto God because there is no question that the Creator of the universe is above us. Our problem comes when we have to give thanks to each other. Whatever position we find ourselves in, we struggle with the concept of putting others above ourselves because our fleshly tendency is to find security by looking for reasons why we are better than, and thus don't need, those around us.
I've heard numerous leaders especially struggle with the ability to say thank you to the people under them. Every time I hear this struggle, the justification is a form of "but that was their job, shouldn't they just do their job?" (or the sentiment, "I shouldn't thank them because a Christian should do things for God, not to get thanks from people", which is essentially the same thought, just candy coated with sugary Christian "goodness"). This view is pride centered because the leader fails to take a position of humility and admit that a job well done made a positive impact on them. If the leader chooses to take this position, they trap themselves into a world view where they feel no gratitude when a task is accomplished well and feel frustration when something is not done well.
Net result: the best outcome is indifference, the worst is frustration, and that is all the people under you will see, indifference or frustration, which is not terribly motivating.
The alternative, the biblical option, is to approach all situations with Thanksgiving. When someone does their job, you praise God for giving you someone to fill that task. The next step, then is to actually go to the person, put them above yourself, and say "Thank you". With an attitude of Thanksgiving, even when people under you miss the mark, you still praise God for letting you see their failings and giving you an opportunity to help guide them and see them grow.
Net result: the best outcome is a grateful heart, the worst is minor frustration but the fulfillment of helping people realize their better selves. The people under you feel like they are cared about and appreciated, which is hugely motivating.
For by the grace given me I say to every one of you: Do not think of yourself more highly than you ought, but rather think of yourself with sober judgment, in accordance with the faith God has distributed to each of you....Be devoted to one another in love. Honor one another above yourselves.~Romans 12:3,10