It is negative by convention. One way to think about it is that it is because the PE at an infinite distance from the gravitational centre is given the value zero. When it falls towards the body the object gains kinetic energy, if the total energy is to be constant the change in PE must be negative to balance the gain in kinetic energy.

Another way is that the definition of potential energy is the integral of F dr (where F is the gravitational force and r is the distance the object falls). As gravity is an inverse square law you are essentially integrating 1/r2 dr which is -1/r. This is where the minus sign comes in...

You might find this page useful:

http://hyperphysics.phy-astr.gsu.edu/hb ... t.html#gpi