The tolerance comes from the same thing that happens with any other drug. Your body gets used to the chemical, and doesnt react the same.

By removing the chemical from your body for a period of time, your body starts getting to the point where it isnt used to the chemical and the reaction happening. Once your body isnt used to it happening, the effects are greater.

Its a problem you see alot in people who use pain medications. I remember the steady climb through pain killers that my step father went... they had to give him larger doses, of stronger medicine as time went on, because his body was so used to the other medicine that it just didnt affect him anymore.