Why we are dividing rate of reaction with respect to a species by its stoichiometry coefficient to find rate of that particular reaction

If there is a reaction like this


$$\ce{ H2 + Cl2 -> 2HCl}$$


I can understand that each individual species (reactant or product) can have different rates and we can compare them like this,


$$-\frac{d[\ce{H2}]}{dt} =- \frac{d[\ce{Cl2}]}{dt} = \frac{d[\ce{HCl}]}{2dt}$$


But I'm having problem with defining the rate of reaction (for the chemical reaction shown above) as the above expressions.


Can someone please explain me why it is like this?


