Humans can have everything they need physically but still be dissatisfied. You may say that this is because we also have an appetite for altruism. We will never be fulfilled unless we serve mankind as a whole. Tech bros have shown that this hypothesis is false. They have everything they could ever want, plus the knowledge that they are using their wealth for the betterment of the species, but are still empty.
Whatever this thing that we desire is, it is not physical. Since having an appetite implies an ability to satisfy this appetite (e.g. we would not feel hunger unless it were possible to eat), there must be something *out there* that is not physical. We may call this God. And because we are able to perceive things that are not physical, there must be something *within us* that is not physical. We may call this the soul.