One thing I can't shake, and it appears already in the second sentence of the quoted letters by Seneca:

"Life is long enough, and it has been given in sufficiently generous measure to allow the accomplishment of the very greatest things if the whole of it is well invested."

What are "great things" and what is the definition of a "well-invested life"? Most of us will live rather unremarkable lives, most likely forgotten in a hundred years or so (at best!). Who says a "good life" means being remembered as long as possible by future humans? Who says the point of living is not to live in luxury and carelessness?

It just seems to me that Seneca assumes the stoic world-view to be the norm. Can anyone help me understand?