Dependent sources removal
Independent sources do not affect the Thevenin resistance because their value is fixed. Voltage sources have fixed voltage and a variable current. Current sources have a fixed current and variable voltage. In a linear system, this fixed value only affects the zero crossing and not the slope. The slope is the resistance. 

With dependant sources, both values are variable, current and voltage, for all the different varieties of the dependent sources. In a linear system, this will affect the slope and intern the resistance of the system. 

A unit of 1 amp/volt is used just to make the calculation easier. Anyone can divide or multiply by 1.