I like the fact that you mentioned "most but not all" You know what I believe? I believe too much of everything is bad, too much of a good thing is bad, there is no two words for it.

Now for the adrenaline junkies,in most cases, but not all(emphasis on not all), Adrenaline junkies develop a kind of tolerance for activities that on a normal day would cause a non-adrenaline junkie to experience a new high(Adrenalin rush), but an adrenaline junkie in most cases have experienced it all, they run out of new highs. As a result, in most cases, they seek alternatives and what are these alternative DRUGS. They start taking in drugs, just to experience a new high, they eventually develop a tolerance for that high and need more of the drug, eventually leading to a drug addiction, its a vicious cycle.