This precious human body, supreme instrument though it is for the attainment of enlightenment, is itself a transient phenomenon. No one knows when, or how, death will come. Bubbles form on the surface of the water, but the next instant they are gone, they do not stay. It is just the same with this precious human body we have managed to find. We take all the time in the world before engaging in the practice, but who knows when this life of ours will simply cease to be?
And once our precious human body is lost, our mind stream, continuing its existence, will take birth perhaps among the animals, or in one of the hells or god realms where spiritual development is impossible. Even life in a heavenly state, where all is ease and comfort, is a situation unsuitable for practice, on account of the constant dissipation and distraction that are a feature of the gods' existence.