Why do diffusion and osmosis occur?
They mainly occur due to the concentration changes at two points
This is the net movement of ions or molecules that move to lower concentration for a higher concentration. molecules move to the place where there is lower concentration of the same molecule., they do not require any kind of membrane
This is the net movement of water molecules from a higher to lower concentration but unlike diffusion, they require a semi permeable membrane.
Please note that osmosis is the movement of 'water' molecules
You asked about the reason.
The reason is the difference in concentrations of the particles at two points respectively.