Why does gravitational potential energy have a negative value?

A point in a gravitational field has an associated gravitational potential. It is defined as the work done required to bring a unit mass from a point an infinite distance to that point in the field. At an infinite distance the potential is zero as it is no longer in the field. Work must be done to move a body from a point in a gravitational field to infinity, where the potential is zero. This implies that the sum of the original energy and the work done is zero and therefore the original potential energy must have been negative.