Why does the pressure of a gas increase when it contracts and decreases when it expands?

If you change the volume of the gas in which it is located, but at the same time the concentration of molecules and the temperature are left unchanged, then the pressure will undoubtedly change. With a decrease in the volume of gas, its pressure will increase, since the number of hits against the walls of the vessel will increase. And if, on the contrary, increase the volume, then the pressure will correspondingly decrease.