Why do submarines and airplanes have a tear drop shape?

The reason for the teardrop hull shape (aeronautical engineers developed it first - naval engineers followed suit later) is that it is extremely strong, yet streamlined enough to allow for the least resistance while traveling through their respective mediums (air and water).

Both use similar methods of control - each have a primary means of altitude/depth control (fairwater/bow planes, wing flaps), trim control (trim planes, tail flaps), and directional control (rudder).

The change to the TD hull design came with the advent of nuclear powered submarines. With increased power comes increased speed potential, and naval engineers searched for a design that would allow them greater speeds underwater. Previous boats had better speed on the surface, since they were essentially surface ships with limited submergence capability. With nuclear power, true submersibles could be built, and submerged speed was paramount. Today's submarines can go 3 times faster underwater than they can surfaced due to their hull designs.

The Bell X-1 rocket plane, which was used by Chuck Yeager to break the sound barrier in 1947, is based on a .50 caliber bullet shape, which was known by engineers of the day to be capable of breaking the barrier. All designs since then have been based on a streamlined shape.

The TD shape is also somewhat spherical in design, and as such provides for much greater strength under pressure at deeper depths. More recent designs (Seawolf, Virginia classes) have gone to a more cigar-shaped hull, which gives it even better strength at depth.