To fully answer the question, we have to establish that there are two aspects of physics involved, mechanical and aerodynamic. The landing gear and wheels act like a spring. An aircraft with zero airspeed, dropped in a level attitude from any height, will bounce to some extent (may be tough to see, but it's there). The other force in play is aerodynamic. This is the result of a relationship between the center of gravity in relationship to the chord line and what happens to the angle of attack when the wheels touch down. As the wing approaches the runway, it has some angle of attack through the air. Generally speaking, when you increase the angle of attack, the lift increases, and the airplane goes up. The converse is also true. As the airplane touches down, if the center of gravity is ahead of the main gear, the nose rotates downward, the angle of attack decreases, and the airplane loses lift and thus tends to stay on the ground. Now, remember, I said generally. If you hit too hard, regardless of the aerodynamics, the airplane will bounce because of the spring like nature of the landing gear. Now, consider the case of a taildragger. Because the CG is behind the main gear, the inertial force tend to drop the tail, INCREASING the angle of attack, and therefore increasing lift, and the airplane appears to bounce. This is why speed control on touchdown is so much more important on taildraggers than tricycle.