>If the rays of light that form the event horizon, the boundary of the black hole, can never approach each other, the area of the event horizon might stay the same or increase with time, but it could never decrease because that would mean that at least some of the rays of light in the boundary would have to be approaching each other. In fact, the area would increase whenever matter or radiation fell into the black hole. Or if two black holes collided and merged together to form a single black hole, the area of the event horizon of the final black hole would be greater than or equal to the sum of the areas of the event horizons of the original black holes.
-A Brief History of Time - Stephen Hawking

Can someone please explain this? Why are the rays of light on the boundary of the black hole not allowed to approach each other? He says that this would happen if the black hole event horizon decreases, but why wouldn't it happen if it increased? Also why is the area of a black hole event horizon more than the sum of the area of two black holes?
I know he came to disprove some of this with Hawking radiation, but why did he ever think this?