how is all fortune good?

The goods of the world, explained fully in Book II, are not really "goods" at all. They are simply inferior goods masquerading as decoration on what is the highest good on earth, the intellectual capacity and soul of human beings. This idea of all the so-called goods of the world - even intangible goods like the love of one's family, or the beauty of Nature - are merely temporal and therefore not able to satisfy the eternal soul, is one of the main "consolations" in the Consolation of Philosophy. The loss anything material or temporal should not be cause of for sorrow, because the only things of value are within yourself and eternal, and can never be taken away.