Anapannasati - Why the breath? Is there a specific reason why the breath is used as the object of anapanasati or to be more precise: Why did the Buddha chose the breath. 

Does it mean then that in daily life our main object should also be the breath?
Seems that moderators are expecting a particular style in these answers, and so deleted my initial answer. I'll rephrase...

The reason that the breath is the most important meditation object is that unlike other objects they are always available. For example, you might forget your TM mantra or misplace your mandala. Other objects of meditation, such as decaying corpses, aren't usually freely available in the UK either :-)