That first hit. Why do addicts take it?

We have been having a sad increase in heroin addicts in NH.  I can understand how hard it can be to give it up once hooked.  The part I don't understand is why people take that first hit.  Surely every sentient creature on planet earth knows that a heroin habit is a life wrecker.  Everyone must have heard that word by now.  And they must believe it.  
  You would think that fear of sinking to the absolute muddy bottom of society, would make people resist taking that first step down the drain.  Surely they would stick with weed or booze before tossing their lives away with heroin addiction.  Apparently they do, but I don't understand why. 
   If we are to have effective public policies against heroin use, they need to focus on preventing people taking that first hit.  Unless we understand why people take that first hit, we won't be able to do much about preventing it.