Why is calculus so interesting?

Because calculus applications are almost infinite. In fact, every branch of science uses calculus : physics, chemistry, Biology, social studies, economics, etc. Calculus is a universal language that can be used to answer bunches of questions. Using calculus, you can solve various problems including the acceleration of planets in orbit, the kinetic energy of a car in motion, the equivalence point of a chemical reaction, the maximal profit a business can make, the lenght of any curved figure or the area and volume of eccentrical shapes, the electric field produced by a charged object, ... Calculus is interesting because you can use it everywhere!