Man born of woman cannot outrun his fate. Better then not to question the Immortals, nor when they have spoken to grieve one's heart in vain. A bound is set to our knowledge, and wisdom is not to search beyond it. Men are only men.
Theseus, in his old age, reflects on life. He sees that there is no escape from the fate that is set before us, and since we cannot escape fate, we should not question it. Theseus comes to a sad view of human life in the end. He realizes that life is sad by definition, because we all are doomed to die. Only the gods live forever, and so man's time is necessarily bittersweet. He feels that we would do better not to question but to attempt to enjoy it while we can.