Letters 70.15

Must I await the cruelty either of disease or of man, when I can depart through the midst of torture, and shake off my troubles? This is the one reason why we cannot complain of life: it keeps no one against his will. Humanity is well situated, because no man is unhappy except by his own fault. Live, if you so desire; if not, you may return to the place whence you came.

Seneca, Letters to Lucilius 70.15, translated by Richard Mott Gummere