There are moral vices in any and every era. Seneca begins the letter with this observation:
You are mistaken, my dear Lucilius, if you think that luxury, neglect of good manners, and other vices of which each man accuses the age in which he lives, are especially characteristic of our own epoch; no, they are the vices of mankind and not of the times.
After describing the various vices through the ages, especially in the era of Cato, Seneca argues that while some may get away with crimes and sins, they will not escape a guilty conscious. While, generally, I agree with this, we have to recognize legitimate sociopaths.
This bad behavior may subside through fear of punishment, but people will most likely not cease to commit these errors on their own accord.
Such things will be done in the future, as they have been done in the past; and the licentiousness of cities will sometimes abate through discipline and fear, never of itself.
Perhaps on-going education and reminders to people will help, but unless we have some sort of moral framework, people will slip back into pleasure and vice.
We degenerate easily, because we lack neither guides nor associates in our wickedness, and the wickedness goes on of itself, even without guides or associates. The road to vice is not only downhill, but steep; and many men are rendered incorrigible by the fact that, while in all other crafts errors bring shame to good craftsmen and cause vexation to those who go astray, the errors of life are a positive source of pleasure.
He goes on to argue that a good reason to not commit vice is simply for the fact of being able to release yourself of the anxiety of repercussions - the fear of getting caught. He agrees with Epicurus, who said,
"The reason that it is no advantage to wrong-doers to remain hidden is that even though they have the good fortune they have not the assurance of remaining so."
While this is perhaps not the best reason (if one is trying to 'make progress') to not commit vice, it is a good reason to give to others who are not lovers of wisdom.
For someone who wishes to avoid troubles, fear and anxiety, it's simply best to be good and remain good. In this way, you easily avoid the feelings of guilt, anxiety and fear stemming from wrong acts.
bad deeds are lashed by the whip of conscience, and that conscience is tortured to the greatest degree because unending anxiety drives and whips it on, and it cannot rely upon the guarantors of its own peace of mind. ... men who hide their sins can never count upon remaining hidden; for their conscience convicts them and reveals them to themselves.