This is a sort of a generalized question and not just referring to the flow of current. This includes fluids and many other such entities.
But why does this flow occur. For example if I consider current, then the definition of potential at any point is the work done by external agent in bringing a unit positive charge from infinity to that point. How can we deduce from this definition that the current will flow from higher to lower potential. In fluids, the fluid flows from higher point to lower point. Why so (referring, again to potential)?
Please avoid any analogies in answering the question.
Best Answer
To answer in terms of electric circuits, we know that electric field $\bf E$ is related to the electrical potential $V$ by
$${\bf E}=-{\bf \nabla} V$$
That means that a positively charged particle in a region with varying potential will experience a force pointing towards regions of lower potential (and a negatively charged particle will experience a force towards higher potentials). In either case we'd describe the result as a current from higher potential to lower potential.
But, it's not correct to say current always flows from high potential to low potential. Every circuit must include some current flowing from high potential to low potential, and some current flowing from low potential to high potential, in order to form a complete circuit. The circuit elements through which current flows from high to low potential consume electrical energy, converting it to some other form (or storing it temporarily). And the circuit elements through which current flows from low to high potential deliver electrical energy to the rest of the circuit, either converting it from some other form (as in a generator or battery) or releasing energy previously stored (as in a capacitor or inductor discharging).
In other systems, there are analogous processes of flow in both directions. For example, water only flows downhill (from higher to lower gravitational potential) because it previously was evaporated by solar energy and was transported to the higher potential region as water vapor and rain.