Why are values for gravitational potential always negative?

This is all down to how gravitational potential is defined. The gravitational potential at a point is defined as the work done per unit mass to move a mass from infinity to the point in the gravitational field. As all gravitational fields are attractive, this means work must always be done to move a mass AWAY from the centre of the gravitational field, but as gravitational potential concerns the work done per unit mass to move the mass TOWARDS the point in the gravitational field, it always has a negative value.