Diffusion Phenomena : Why does diffusion occur?
The short answer
They don't.
The long answer
Molecules are very fast. For example, a single molecule in the air around you can be as fast as 500 km/h. The reason that you do not feel them is because they are too small.
Also, you need to understand that there are very, very, very many molecules in a &quot;small&quot; volume.
So, until this point, you have many molecules which travel very fast.
Suppose a gas is divided to high and low concentration areas. The area with high concentration would have more molecules, and let's say 50% of them is travelling towards the area of low concentration (they all move randomly, and in a straight line unless collision happens).
Let's say, 50% of the gas molecules in the area of low concentration is also travelling to the area of high concentration.
Then, you would have a net movement of molecules from high to low concentration.
The keyword here is net. Some molecules do go from low to high concentration. However, they are outnumbered.
After they reach equilibrium, there is still gas molecules travelling between the two regions, just that they are equal in number.
Extra
Of course, because my explanation being not very formal and strict, there would be some errors in my explanation, but I hope that you understand it.