I can give you a saying of your friend Epicurus and thus clear this letter of its obligation. “It is bothersome always to be beginning life.” Or another, which will perhaps express the meaning better: “They live ill who are always beginning to live.” You are right in asking why; the saying certainly stands in need of a commentary. It is because the life of such persons is always incomplete. But a man cannot stand prepared for the approach of death if he has just begun to live. We must make it our aim already to have lived long enough. No one deems that he has done so, if he is just on the point of planning his life. You need not think that there are few of this kind; practically everyone is of such a stamp. Some men, indeed, only begin to live when it is time for them to leave off living. And if this seems surpising to you, I shall add that which will surprise you still more: Some men have left off living before they have begun.

As a matter of fact, pretty much everything Seneca ever said about death is a gold mine in this regard, especially his essay, On the Shortness of Life.

http://www.forumromanum.org/literature/seneca_younger/brev_e.html