In chapter one of Heart of Darkness what prompted marlow to tell this story?

Answer 1

Though not having the propensity for many things, he did have a propencity for spinning yarns. The river Thames and the on coming of brooding gloom in the evening brought one forth a yarn of the ancients and it lead him to speak of his own experience and then his appointment as the Captain of a Riverboat the plied the river that lead to the Heart of Darkness.

Answer 2

It was all that he had left.