Why does a negative charge move opposite to the direction of an electric field?

An electric field is defined by the force it exerts on a unit positive charge. By convention, the direction of the electric field is chosen to be the same as the direction of the force exerted on a positive charge.1

Because the force exerted on a negative charge is always opposite to the force exerted on a positive charge at the same point, a negative charge experiences a force in the direction opposite to the electric field. Consequently, when a negative charge is placed in an electric field, it moves in the direction opposite to the field lines. This behavior is also reflected in the relationship between force and electric potential: while a positive charge is pushed toward regions of lower potential, a negative charge is subjected to a force toward regions where the potential increases most rapidly.123