The action of shielding is a deep concept that stems from an ability to both defend and offence. A shield can be used for both protection and destruction and it's this nature of the shield that can confuse many people.
Shields act on the will of the wielder and it was always intended as a battle item. Make no mistake, those who wield shields are not those who wish to remain passive. They are participants in conflict.
When you carry a shield, you may think twice about your intentions and the nature of said shield.