It's not so much a resistance ("resist" implies that your body is trying to reject it) as it is a tolerance (your body comes to tolerate the drug and its effects).
As with pretty much every drug, the more you use it, the more of it you need to use, because your body starts adapting to having it in such a quantity with such a regularity.
(This is also one of the reasons why withdrawals happen - once your body gets used to having something, taking it away tends to have detrimental effects.)
So to get the same (or more) effect once your body has built up a tolerance, you either need to use the drug more often, or in larger quantities (or both).
(For me, caffeine makes me sleepy. With milk [i.e., caffè latte, although I seem to tolerate half-and-half perfectly fine], it makes me nauseous, what with my being mildly lactose intolerant.)
-- Griffinhart