Why do we define density as mass per unit volume.

Imagine a room with only one bed one chair and one table
Situation one:-
Only one people is staying.
Situation 2:-
two people is sleeping in bad, while one is sitting there. Another person is moping the floor whereas one is sitting in chair with a earphone hearing music.
So much crowd, right?
WAIT !! Still there are two more person sitting above the table.
Situation 1 = less density
Situation 2 = high density.
Now let's try to apply some mathmetical concepts to it.
We know Density is mass divided by volume.
in the above mentioned cases mass refers to the number of people and volume is the size of the room. In situatuion 2 density increased as the mass increased while volume remain the same.
Now if you change the room with the auditorium the density will drastically change, isn't it? As the volume is increasing.

The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ (the lower case Greek letter rho), although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume: