Yes. And you answered yourself: ADDICTION. That's the reason right there. It's hard to break an addiction but if you recognise you have one at least you know that you can't trust your logic, no reasons to fall back into the addictions are valid no matter how good they might seem. If you identified your addiction you have to prepare your NO! response to every thought that is connected to it. Be it drugs or alcohool or people or even internet, just say NO whenever the idea comes to you. Be strong! Be safe! Don't let yourself be a victim!

Because your body gets addicted to heroin and needs it to get through the day, same premise for anything I guess.

They don't have anything else to do.

i'm not sure if i know what you are talking about but its an adiction.

Addiction.

because they are addicts.. what ever they get from it is rewarding enough for them to keep doing it even tho its not good for them.

The reward is (deemed to be) greater than the risk and/or consequences.

what are you talking about

Life is short.