Why does a positive test charge move in the direction of an electric field?

The direction of an electric field is defined by convention rather than by a physical law of nature. Because positive charges naturally repel one another, the direction of the force exerted on a positive test charge is chosen arbitrarily to represent the direction of the electric field itself.1

In physics, the electric field strength ($E$) at any point is defined as the Coulomb force ($F$) exerted per unit positive test charge ($q$), expressed as $E=F/q$. Since the field is defined by the force acting on a positive charge, a positive test charge placed within that field will experience a force—and therefore move—in the same direction as the field lines. Conversely, a negative charge experiences a force in the opposite direction of the electric field.1