Why do we have to ask for protection daily?

[BrownEye]

I found that if i am not in any workings that entities may want to block, my shielding holds at a steady level. My problem is that when my shield is up i am fully unaware of attacks, which seem to have the effect of weakening it. For the most part, i just do not think of it, which lessens it like not watering a plant.