The given blank can be filled with plate tectonics.
The theory of plate tectonics states that the outer shell of the Earth is differentiated into many plates, which glide over the mantle, that is, the rocky inner layer above the core. The plates function like a rigid and hard shell in comparison to the mantle of the Earth. The tectonic plates give rise to earthquakes and volcanoes.
I think the passage is trying to tell the reader that they shouldn't lose faith in people/animals just because they are elderly/weak when they are loyal to you because it will hurt their self-esteem.
