The reaction $2 NO_2$ $\longrightarrow$ $N_2O_4$ is the result of simple collisions between molecules of $NO_2$ .
You see molecules of $NO_2$ gas in an enclosed space.
The collisions don't happen very often, because there are few molecules.
You see twice as many molecules in the same enclosure and the same conditions.
Every molecule is twice as likely to make a collision, because molecules are arranged closer :
Number of molecules x 2
For every molecule: Number of collisions x 2
...
$4=2^2$ , $9=3^2$.
What is true for the number of collision in this enclosure is also true for the number of moles reacting per volume unit, therefore for the molarities!
The reaction $2 NO_2$ $\longrightarrow$ $N_2O_4$ is the result of simple collisions between molecules of $NO_2$.
