It's all to do with proportion.
Boyle's Law has the pressure of a gas inversely proportional to its volume. This means that as the volume of a fixed amount of gas decreases, its pressure increases, and vice versa, provided there is no change of temperature. This seems logical, because as a gas disperses (volume increases) the pressure it exerts drops. It also means that the product of pressure and volume is constant.
Charles Law takes the volume and temperature as measurements, and keeps the pressure constant. This, too, is logical. If you heat a gas its volume increases and if you cool it the volume decreases. So volume is directly proportional to temperature. But there's a catch. The temperature isn't Fahrenheit or Celcius, because these are two different scales. Absolute temperature is what has to be measured which has its zero at absolute zero when the molecules of the gas are at rest. The temperature is in degrees Kelvin, which has is zero at -273 degrees Celcius. This means that all temperatures must be converted to degrees Kelvin, otherwise it has no meaning. So V is proportional to T where V=volume and T=absolute temperature.