Lucilius' friend, Flaccus, died and Seneca is writes about grief.
Stoics are not without emotion. The excess and deficiency are perhaps not ideal. Therefore, when grief comes as a result of a loss of a friend or loved one, we should neither grieve with excess nor should we lack grief at all.
I would not have you sorrow more than is fitting. That you should not mourn at all I shall hardly dare to insist ... Let not the eyes be dry when we have lost a friend, nor let them overflow. We may weep, but we must not wail.
Some people may truly be deeply grieving, but if they do not cease at some point, then there may be deeper issues or perhaps it is for show.
It is because we seek the proofs of our bereavement in our tears, and do not give way to sorrow, but merely parade it. No man goes into mourning for his own sake. Shame on our ill-timed folly! There is an element of self-seeking even in our sorrow.
Later in the letter he write,
The reason why they lament too unrestrainedly at such times is that they are afraid lest men doubt whether they really have loved; all too late they seek for proofs of their emotions.
To grieve is human, to grieve excessively is folly. It would seem excessive grief is no longer about the lost loved one but about the griever!
Therefore, we ought to grieve, let our grief be about the person who we lost, and let us move on contemplating the memory of the one lost.
Let us see to it that the recollection of those whom we have lost becomes a pleasant memory to us. No man reverts with pleasure to any subject which he will not be able to reflect upon without pain. So too it cannot but be that the names of those whom we have loved and lost come back to us with a sort of sting; but there is a pleasure even in this sting.
And if we contemplate the death of our friends, similarly to how we contemplate our own death, we will enjoy the company of our friends while they are yet with us.
To me, the thought of my dead friends is sweet and appealing. For I have had them as if I should one day lose them; I have lost them as if I have them still. ... Fortune has taken away, but Fortune has given. Let us greedily enjoy our friends, because we do not know how long this privilege will be ours.
Seneca then goes on to remind us of the importance of friends and to not only have one, but many.
If we have no other friends, we have injured ourselves more than Fortune has injured us; since Fortune has robbed us of one friend, but we have robbed ourselves of every friend whom we have failed to make.
And when we lose a friend to death, we ought to seek a new one.
You have buried one whom you loved; look about for someone to love. It is better to replace your friend than to weep for him.
I don't know how precise Seneca is talking here. Would he include spouses in this discussion? I know some men and women, who after having lost their spouse to death, refuse to remarry because it would violate some trust or commitment to their spouse. Even some children do not want their parent to remarry after having lost the other parent. But I think Seneca makes a very valid point. We humans need friends; we need social interaction and we need close friends. If we lose one, we ought to find new ones. Perhaps that does not mean getting remarried, but that does not mean slamming the door shut on the proposition of another marriage.
His next point is interesting.
the most shameful cure for sorrow, in the case of a sensible man, is to grow weary of sorrowing. I should prefer you to abandon grief, rather than have grief abandon you; and you should stop grieving as soon as possible, since, even if you wish to do so, it is impossible to keep it up for a long time.
It seems he is saying that we need to actively observe and be aware of our grief and approach our sorrows rationally. Acknowledge that it is human to grieve; then grieve; then reconcile and move on. But, don't let time cure your grief. He proposes active resolution, rather than passive abandonment. Perhaps to use a dental analogy. We should deal with a toothache, rather than letting time do it's work. If we deal with a toothache, then a dentist can help us resolve it and we can keep our tooth. But if we simply let time pass, we eventually get over the toothache, but lose the tooth in the process.
In sum, practice contemplation of death - yours, your friend's and death in general.
Now is the time for you to reflect, not only that all things are mortal, but also that their mortality is subject to no fixed law. Whatever can happen at any time can happen today.