Why shouldn't Arjuna grieve for the body?

In the 25th verse of chapter 2 of Bhagavad Gita for happiness, Lord Krishna says that the soul is invisible, immeasurable and unchangeable — knowing this Arjuna should not grieve for the body. The magnitude of the soul is so small for material calculation that it cannot be seen through a microscope. There are many things that we accept on the ground of superior authority, and Lalwani elaborates on the same.