HOW THE VOICE WORKS
The voice starts out as a tiny oscillation caused by air busting through your vocal folds. This oscillation is magnified and shaped by the cavities and articulators of your ear, nose and throat.
A sound wave is a cycle of compression and decompression (or rarefaction) caused by a mechanical force of some kind. Basically by chopping up little bits of air your vocal folds create a mechanical wave of vibrations that propagate through the air and are interpreted by your ears as sound.
Take a look at the wave form below:
If this were a recording of silence then it would be a straight line. Each wiggle (or wave) describes a sound. The closer the waves are to each other (i.e. faster) the higher the pitch. The taller/lower the wave is, the louder it is.
What is true of the sound wave is also true of the mechanical wave used to create it. Thus, when you want to talk or sing higher, you vocal folds must open and close faster. When you want to talk or sing louder, your vocal folds must open with more force/energy.
It is no wonder then, that vocalizing high and loud (without solid technique) is the fastest way to destroy your voice.