Why the missle and rockets always follow the parabolic trajactory while launched from the ground?

It because the there are two components in a two dimensional motion of a particle.The horizontal and the vertical.The horizontal component contribute only linearly according to initial velocity vh and distance travelled is vhxt(t=time of travel).But the vertical component is influenced by Accleration due to gravity the displacement due to which varies according to square of time.This is the reason for parobolic trajectory 0f the type y=at-x t squared which is of the parobolic form when plotted in two dimensions