But friendship is precious, not only in the shade, but in the sunshine of life, and thanks to a benevolent arrangement the greater part of life is sunshine.

The line contrasts two common “tests” of friendship: adversity (“shade”) and prosperity (“sunshine”). Jefferson’s point is that friendship should not be valued only as consolation in hard times; it is equally meaningful as a companion to joy, success, and ordinary well-being. The closing clause—crediting a “benevolent arrangement” for life’s prevailing sunshine—adds an Enlightenment-tinged optimism (and a hint of providential order) that frames friendship as something meant to be actively enjoyed, not merely relied upon. In a quotations context, it functions as a corrective to the cliché that friends are chiefly for crises, emphasizing shared happiness and sustained affection.