I have also heard, a number of times now, that a lot of smart people use drugs because it "makes people more interesting", which is probably code for "I numb my own brain to be able to strike a casual conversation". (My primary example would be having this as a direct quote by Christopher Hitchens. I seem to also recall that there was a House MD episode of a genius putting himself on a controlled substance to be a little more numb because he would otherwise scare away the girlfriend that he loved with his rambling mind.)

So yes, some people might be searching for a thrill because what they are supposed to do does not deliver that. But others just really want to "be normal" and drugs are a quick way to get there, strangely.