Seneca suffers from asthma and according to what he says, physicians of the day called asthma "practicing how to die" - which some have also called philosophy!
He notes it would be
absurd to take delight in such supposed restoration to health, as it would be for a defendant to imagine that he had won his case when he had succeeded in postponing his trial.
Rather than taking delight that he's overcome a bout with asthma, Seneca reassures Lucilius that he has "never ceased to rest secure in cheerful and brave thoughts." He has long pondered the before and after of living, and arrived at the conclusion he's already lived death before he was born!
would you not say that one was the greatest of fools who believed that a lamp was worse off when it was extinguished than before it was lighted? We mortals also are lighted and extinguished; the period of suffering comes in between, but on either side there is a deep peace. ... it has both preceded us and will in turn follow us.
And because he has spent time contemplating death, he claims he is ready for it and won't be frightened when it comes and will go willingly.