The force caused by air resistance is proportional to its velocity. The higher the speed, the more force you need, why? because it is "moving" more air, and that is energy. How the surface is two dimensional and motion is exerted perpendicularly to the amount of air moved or impacted drop is measurable volume per unit second. The energy required to move the particles within this volume (subsequently dispersed by butterfly effect in contact with adjacent surfaces) comes from the kinetic energy, ie, its velocity, which in turn comes from the gravitational potential energy (as that advances in the gravitational force field is gaining speed at a rate of g, for the small scales can be considered constant, but you should realize that a field is ALWAYS unchanged "small" which is their difference, these are practical considerations, more than theoretical). The higher the speed, the volume of molecules per second and higher colliding energy transfer due to their impact and friction layers. The impact between the surface and the air is much more complicated than a simple impact because they involve different particles in motion, but you can get an idea intuitively. Since energy can neither be created nor destroyed, it just flows. On one side we gain speed thanks to the potential energy of gravity, but this gain is constant and on the other side have a loss because the consequences of interacting with other particles and dissipate energy, which is proportional to speed. As you can see the speed plays the role of intermediary in the process, and there is an equilibrium in which a special rate equals this "transfer" of energy, which is the terminal velocity. At this speed the energy dissipated is equal to the energy supplied by the gravitational field and hence remains constant.
Sorry for my English, I'm Spanish and I'm not very good with languages.