The letter starts of emphasizing the importance of conducting self-retrospectives, so that we may learn from our past.
Seneca advocates transparency and constant introspection.
we should live, – as if we lived in plain sight of all men; and it is thus that we should think, – as if there were someone who could look into our inmost souls; and there is one who can so look. For what avails it that something is hidden from man? Nothing is shut off from the sight of God. He is witness of our souls, and he comes into the very midst of our thoughts – comes into them. ...
I shall keep watching myself continually, and – a most useful habit – shall review each day. For this is what makes us wicked: that no one of us looks back over his own life. Our thoughts are devoted only to what we are about to do. And yet our plans for the future always depend on the past.
Then he delves into the topic of drunkenness.
I've never been drunk and I don't understand the desire for people who drink until they are drunk. In high school, I generally avoided the crowd that smoked weed and drank. While I was living in Guatemala, I saw plenty of drunks. Two examples stand out - one repulsive, the other graphic and tragic.
The first was of a man who was attempting to walk home, but who was so slammed, he passed out in the middle of the dirt walking path, just before a foot bridge. He was lucky enough to have avoided smashing his head on something hard enough to split his head open. But, after he passed out, he wet himself and vomited.
The second was of a man who was so smashed, he could not hold onto the metal bar while standing in the back of a truck (jalon). Therefore, he stumbled backwards, out of the back of the truck, hit the ground and cracked open his head and bled out before help could arrive.
As for Seneca, he makes a good argument, simply stating that the sage will not get drunk.
the sage ... always stops short of drunkenness.
if you wish to prove that a good man ought not to get drunk, why work it out by logic? Show how base it is to pour down more liquor than one can carry, and not to know the capacity of one's own stomach; show how often the drunkard does things which make him blush when he is sober; state that drunkenness is nothing but a condition of insanity purposely assumed.
The rest of the letter are the many examples of how drunkenness usually brings out the worst vices in us.
If you or a loved one suffers from alcohol addiction, seek help. Here's one resource to consider.