She (Philosophy) tells Boethius that luck always changes, and that he shouldn’t be surprised by this. That’s the nature of luck. It is fickle. Luck is random. There is no guarantee that because you are lucky today, you will be lucky tomorrow.
Mortals are foolish to let their happiness depend on something so changeable. True happiness can only come from inside, from the things that human beings can control, not from anything that bad luck can destroy.
The message is that riches, power and honour are worthless since they can come and go. No one should base their happiness on such fragile foundations. Happiness has to come from something that is more solid, something that cannot be taken away.
Philosophy’s answer is that he will find true happiness in God, which she seems to describe as the pure Form of goodness.