Do not confuse the boss with your friend

So I was talking to my boss and I was telling her something I have always known to be fairly obvious: Bosses and subordinates cannot truly be friends. Now I know that people will say: That is SO not true! I tell my boss everything, we go out to the bar, we have had dinner at each other's house... This may all be true but let me ask you this: Could you tell your boss that you are thinking about going to work somewhere else long before you make the decision to do so? Now, could you tell your best friend? The answers to these questions are obvious.
The reason you do not tell your boss you are thinking about working somewhere else is because THEY ARE YOUR BOSS and they cannot never not be your boss. Once they know something about an employee that affects the company they have to act in a manner to protect the company. This does not change no matter how many beers you have together.
Two more things: everyone already know this and it is a good thing. People always keep secrets from their boss that their good friends could write books about. They do this to protect their jobs and maybe that is the way it should be. It should be this way because if a company and its employees do not protect themselves, there will be no water cooler or employees to gossip around it.