Boyle's law states that for a fixed amount of gas in an enclosed system at constant temperature, pressure and volume are inversely proportional. In other words, when one of the two variables is changed, the other variable is changed in the opposite direction. That is why both variables are on the same side of the universal gas law equation PV=nRT. So if the volume increases, the pressure decreases. This makes sense if you think of a balloon of any gas. If you could stretch out the balloon without adding more gas to the inside, the pressure that the balloon exerts on the gas would lessen.