Why DC series motor should be started with load?

In any DC motor, the speed of the motor is inversely propotional to the flux and directly proportional to Back EMF . 

When starting a series motor the current flowing will be high, but the flux that the motor produces will be moderate since the series winding has less number of turns. 

Hence, if u start the motor without load the speed will increase. 

As speed increases the back emf increases. 

This decreases the current through the series winding and hence flux decreases and the speed further increases. 

Theoretically speaking the speed becomes infinite. But practically the motor will accelerate to very high speed , which will damage the motor. 

But if you start the motor with the load, you are actually reducing the starting speed hence the motor runs safely. 

Since series motors has this property it is used in trains to pull high load.

circuit diagram of dc series motor