relationship exist between force and acceleration is directly proportional.to definition of force.
when a body apply a 1 n of force on a body of mass 1 kg it produces acceleration of 1 m/s² in the direction of force.
→[tex] f \propto ma [/tex]
→[tex] f = k \: ma [/tex]
where k is constant of proportanility.
its just a superstition made by our past ancestors.theres no further reasons