I read a good book once by a defector from the Soviet military, explaining the lessons he'd learned as an officer on the other side of the iron curtain (this one: http://en.wikipedia.org/wiki/Inside_the_Soviet_Army).

In it he described one thing he'd learned about how to lead people:

Always try to remember the golden rule of controlling others -- NEVER PROMISE ANYONE ANYTHING! If you are able to do something for another person -- do it, without having made any promises.

From this first rule there follows a second -- NEVER THREATEN ANYONE!

You can punish someone and, if you consider it necessary, you should do so. But promises and threats simply weaken your authority as a commander.

(The full text of that chapter from the book is here: http://militera.lib.ru/research/suvorov12/08.html)

This is true in civilian life, too. Promises and threats are just wind that people blow when they can't do what they really want to do. Leaders with real power don't threaten to fire someone; they just fire them. They don't promise to give raises; they just give them.

People understand this, and if you lean on promises and threats, it makes them subconsciously evaluate you as weak, ineffectual. Someone who it's not really necessary to listen to.