Why is it better to press down at the end of the stapler then pressing at the middle to staple paper together

Explanation:

Force pushing down onto the staple dispensing mechanism (I have no idea what else to call it) is how it operates.  So the more you can push down onto that area the better, to a point of course but we'll assume you aren't trying to break it.  

You can think of this in two ways.  The force being closer to the part that needs the force is better, so if you move it further away there will be less force there, or you can look at the top of a stapler as a lever, with the hinged end as the pivot, so the further you are away from the pivot the less force you have to exert  to get the same amount of force pushing on the other end of the lever.  

Let me know if that doesn't quite make sense.