As was stated in the other respones & links, gravity is what creates the black hole. A small star like our Sun has enough mass (and therefore, enough gravity) to collapse down to a white dwarf star when it "dies". But at that point, gravity cannot overcome the resisting pressure of the atoms. A larger star has more mass, and therefore more gravity. It can compress atoms down further into neutrons (creating a smaller neutron star). But the neutrons cannot be compressed any further. An even larger star has so much mass/gravity that even the neutrons cannot stop the compression and the entire mass is crunched into a singularity. So much mass in such a small space means that the gravitational force is HUGE when you get close to it. It's so huge, than not even light can escape. That is a black hole.

So in the analogy, the flat slob not only cannot lose weight, but his body cannot support his own weight and he is crushed into a point.