The SI unit scales are chosen to fit together to avoid “respectable” scientific calculations.

To heat one milliliter (1 ml) of water one degree Celsius (1 °C) you need one calorie (1 cal) of energy.

Also the dimensions of one milliliter, is one cubic centimeter (1 cm^3), and that amount of water weighs 1 gram (1 g).

Thus 1 liter of water needs 1 kcal of energy to heat up 1 °C.