This one the most fundamental questions in physics. We really don\'t know the answer. The fact remains that in all experiments performed there is no evidence for charges other than that of the electron, or integer multiples thereof. Modern quark theory speculates that the proton is made up of objects with fractional charges of 1/3 and 2/3 that of the electron. This theory explains much of the experimental data in High Energy Physics. However, no free quarks have been observed. Paul Dirac speculated that if there existed an object called the magnetic monopole then charge quantization would be automatically required by quantum mechanics. Unfortunately, we haven't seen any magnetic monopoles either. So, what's going on? I wish I could tell you. See http://en.wikipedia.org/wiki/Magnetic_monopole
Of course, assuming those e/3 charges exist, it's still quantized, just with a different quantum. So there's still some mystery. Mike W.
(published on 02/20/09)