All the Dobermans I have know have been pushy and stubborn. Why is this?

Well, if you let a Doberman rule he will. The Doberman is sure of himself and knows someone needs to be in charge. Likely, the Dobermans you have met were not thoroughly trained or lacked a strong alpha leader.

The sad thing is this stubbornness is the same part of the Doberman's temperament that, when trained, makes him so obedient.