I have a couple degrees in Biology but never took a pharmacology class. This tolerance effect doesn't just happen with drugs, it can happen with almost anything, including food and even having fun. For example, if you quit eating sugar for a month, and then have something sugary, it will take much sweeter because your sweet taste buds have been upregulated. If you do the same 'fun' routine all the time, after a while it will not be as fun anymore because the lack of novelty will not stimulate your dopamine receptors as much. Conversely, if you drink a lot of alcohol daily, your GABA and probably other receptors will be downregulated because there is too much 'ligand' or starting material that hits the receptors.

The way I look at it from a simplistic point of view, is that your body is trying to get a result that it wants (not necessarily that YOU want, but that it wants):

A x B = C. A is the drug, B is the receptor, C is the result. If the result needs to be a number of 100, and your body has '10' worth of receptors, then it needs 10 of the drug because 10x10=100. But if you add 20 of the drug, now 20x10=200 and that's too much, so your body downregulates B (the receptor), so now it's 20x5=100 again.

But why does this happen with some drugs and not others. Someone can take the same blood pressure drug for years and not develop a tolerance. There is a big controversy about benzodiazepines causing tolerance. The sedative and hypnotic effects can have a tolerance but the anxiolytic (anti-anxiety) effects actually do NOT cause a tolerance (if you don't believe that, look it up in PubMed, search benzodiazepine tolerance). But exactly why that is, I don't know. There are some people that can take the drug for 10 years on the same exact daily dose and be functional; they are not considered to be abusing the drug. The drug keeps working for them; they have their anxiety under control, at least 99.9% of the time. Other people, either for psychosomatic/dependence issues or because they abuse the drug or mix it with other drugs, feel the need to take more and more of it, to the point that on paper they are taking way too much and yet they say they are more anxious than ever. Obviously a controlled substance like that is causing brain changes, but why such a large difference in response from one person to the next? And why can one person get off that type of drug and be fine, and another tries to get off and they end up with all kinds of strange neurological problems that they didn't have before?

Either the drug works or it doesn't. If it causes a tolerance, then everyone would need to take more of the drug, but they don't. Only some people. And not all of those that feel the need to take more are drug addicts. I find that very interesting.