There are a lot of answers to this question. We are all wired with the ability to become addicted to something. It's just a matter of coming across what does it for you. Cocaine, Opiates, Alcohol, but don't forget about about coffee, cigarettes, food, sex, etc...
Some people have addictive personalities while others do not.
For me, I just loved the feeling it gave me. The downside of becoming addicted is when the brain no longer produces endorphins on it's own because you have provided so much from an external source (pills, IV, etc..) Without endorphins your entire system goes out of whack. Endorphins are involved in respiration, naseau, pain modulation, hormonal regulation. So when your brain stops producing endorphins on it's own and you try to stop taking the external source you have made your body accustomed to getting, you become extremely sick for a few days. What we call "dope sick". Your body enters such a confused state as it is trying to turn it's natural production of endorphins back on. It is one of the most aweful experiences to go through. Ive been trying to prepare myself for it in another attempt to quit heroin. But all it takes is a glimmer of being sick and the knowledge that getting a fix will make you feel better and youlose that battle every time. You will do whatever it takes to avoid being sick like that.
After a period of time abusing opiates, you totally rewire your brains' reward/pleasure system. You're brain forgets what it mean to experience natural highs and will always want that high you experienced on opiates. You will spend the rest of your life trying to experiece that first high again.