Answer
See the detailed answer below.
Work Step by Step
We know that the gravitational potential energy between two objects is given by
$$U_{g,12}=-\dfrac{Gm_1m_2}{r}$$
Now why this is negative?
Recall that we need to find a position at which the gravitational potential energy between these two objects is zero.
We chose that position to be at infinity.
$$U_{g,12}=0\tag{when $r=\infty$}$$
Now if we let both of them move freely they will move toward each other which means that the distance between them decreases. This is also one reason of why it is negative.
Recall that the gravitational force is a conservative force and it is an attractive force that makes both objects speed up toward each other until they reach their maximum kinetic energy when the distance between them is zero.
