That's not to say that you can't like your boss. Or lend him money. Or think he is going to use the old "shoe wedge" on the links when you aren't looking. I am not talking about trust as a character trait. I am talking about understanding that his best interests don't always lie with your best interests.
"Bing" who writes business humor for Fortune put it best:
Never forget that your boss will happily eliminate your high paying job in order to keep his higher paying job.
And that is the crux of the matter. Your boss is answering to higher ups and is concerned about you only so far as it makes him look good. If something that makes him look good happens to also be something that makes you look bad, guess what the chances are that it will happen? If a higher up asks him to do something that conflicts with a request you made, where do you think he will land?
This is all put into perspective that part of your job is to make your boss look good (See Rule 15). This is the flip-side of the coin, and a reminder that the reverse is not true.