Why does heat transfer from a hot body to a cold body?

Nature always try to create a balance in the system. Technically it is known as equilibrium. Thus nature tries to create thermal equilibrium between hot and cold bodies. Everything is ready to go from high to low because of the availability of the gradient. The atoms in the hot bodies have higher kinetic energy than those of the cold bodies. Thus to maintain thermal equilibrium, the atoms of higher kinetic energy tries to move and collide with the atoms of low kinetic energy.  Thus heat transfers from a hot body to a cold body.