What is the main conflict in Letters from a Stoic Epistulae Morales Ad Lucilium by Seneca?

In this (abridged) collection of letters, Seneca presents his moral teachings, heavily influenced by the Stoic school of philosophy, to Lucilius, a young man who belongs to the same upper echelon of Roman society as Seneca. Though the letters cover a great breadth, one theme in particular permeates his thought: true happiness is achieved through inner peace and cannot be taken away by the winds of fortune.