When you are braking you generate a lot of heat and the brake fluid is created to withstand a lot of heat and has a very high boiling point. 
The brake fluid reservoir sits on the upper drivers side under the hood.  What happens inside the reservoir is it sweats and water gets into the brake fluid.  The water in the fluid lowers the boiling point of the fluid.  If you were to use your brakes a lot going through the mountains you could boil the brake fluid and you would lose the brakes.  This could be pretty scary.  Most shops have either a strip or another tool to check how much water is in the brake fluid.  A general guideline for changing the brake fluid is 50,000. 

The brake fluid was leaking out of car on a program I was working on and someone asked me “what happens when you lost all of the brake fluid…………..do you lose the ABS brakes?”  I looked at this person all surprised and said when you lose your brake fluid you lose your brakes!