This cliché was evolved around the idea that, we cannot assume what we want till it is in our hands and is completely our own. This can refer to a number of things from physical objects, to actual events in life or dreams and goals. All we can really do is hope things will happen or wish for the result we want.
In the video above a bike rider links the cliché to an actual event. Coming to the end of the race he presumes he has the race "in the bag", its all his and he has the title of a winner. However in that moment when he may have been imagining the trophy and his success, a fatal mistake causes him to loose and inevitable shattering his dreams.
If we refer back to the saying and think literally using the example of a chicken farmer, we can see how he will presume the possibility of loss or a negative outcome. So the loss of a few chicks allows him to measure up the scenario in reality even though he would prefer to hatch all the eggs.
The saying constricts and limits us not too expect much. From this we avoid disappointment and usually try to picture the worst. In my opinion we have taught ourselves that life can really be out of our control and therefore we don't live our dreams instead we imagine them and the various ways that they could one day be a possibility.