In our normal lives, there is a hierarchy of how we get our reward. For example: You desire money, which opens up a possibility for material possessions or buying certain kinds of services/experiences. The external objects have no intrinsic value. Its how they make you feel. Sometimes the external object works to satisfy concepts held in our minds and this provides a much greater component of the feeling.
Say a person wants to possess a BMW. Some of his reasons could be that it represents a particular strata of society, the cream of it(concept)...an elite possession(concept)...only the rich possess(concept)....defines luxury(concept)...a comparison with all the people around you could give superiority - this superiority is actually not because you have something they do not possess...its because you have something that the common public considers highly desirable(concept).
I feel that satisfaction of the concept gives much greater pleasure than the pure visceral component of it. The visceral component or the concept getting satisfied creates a sensation in the body and it is judged. The sensation+judgment = Feeling(pleasant/unpleasant etc.).
So this long hierarchy leads you to the feeling which is the reward. Drugs cut through/pierce this hierarchy straight down and directly give you the feeling you are looking for without any of the in-between part. For an analogy, they act like wormholes in the space time fabric. Thats one of the reasons why it may be so baffling and disorienting to a person who has never thought in this perspective. And i'm still talking in the intellectual plane, leave alone the actual experience. It gives you what you are looking for all this time and you may never achieve that feeling ever in real life and in that intensity.
But here are some of the catches. Since we understand something only from its opposite, you real life will seem really low in comparison to that. Because you've witnessed that high, all the events in your life will seem extremely meager. Nothing will seem worth pursuing besides the drug itself, because nothing can give a reward that is even close to that. In other words your sensitivity to real life would get drastically reduced due to this new benchmark that has been set. All your experiences would now be judged according to this benchmark.
Things like meditation or sensory deprivation do exactly the reverse. They heighten your sensitivity to make rewards appear larger and clearer, resulting in happiness. The way I look at it is, both these methods do show you a lesser known secret of life. They may be equally overwhelming/transformational/disorienting(for a normal way of life). But they nevertheless represent polar opposites in the way they are done.