Seneca felt that happiness comes from within. It is impossible to be happy while relying on luck and external circumstances. In other words, happiness shouldn’t be conditional on anything outside of your control.
Do you ask what is the foundation of a sound mind? It is, not to find joy in useless things. I said that it was the foundation; it is really the pinnacle.
We have reached the heights if we know what it is that we find joy in and if we have not placed our happiness in the control of external circumstances.
Seneca acknowledges that feeling joy that doesn’t come from external circumstances may not come naturally:
Above all, make this your business: learn how to feel joy.
I do not wish you ever to be deprived of gladness. I would have it born in your house; and it is born there, if only it be inside of you. I mean from your very self, that which is the best part of you.
Furthermore, the kind of joy that arises from outside tends to lead to sorrow:
Pleasure, unless it has been kept within bounds, tends to rush headlong into the abyss of sorrow.