Moral Letters to Lucilius Section 10

Quare? inquis, desiderat enim explanationem ista vox. Quia semper illis inperfecta vita est. Non potest autem stare paratus ad mortem, qui modo incipit vivere. Id agendum est, ut satis vixerimus. Nemo hoc putat, qui orditur cum maxime vitam.

car le mot demande explication. – C’est qu’alors la vie est toujours inachevée ; or qui peut se tenir prêt à mourir, s’il ne fait que la commencer ? Il faut agir de telle sorte qu’on ait toujours assez vécu : et nul ne s’en flatte au moment où il ébauche son existence.

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.