I think surroundings make a big difference in someone's addiction. Changing one's environment is a big step in getting into recovery.
It depends on your personality type. Generally people want to belong and will go along with the group for fear of being rejected, and it takes a very strong personality to go against the others in the group.
I first tried marijuana because ALL of my friends around me were doing it. I wanted to belong, I wanted to be able to talk about my high when they were talking about theirs. I didn't want to be the outsider when they all went off to smoke.
After I got pregnant with my first child, it was like a light switch went off in my head. i couldn't keep smoking like i had been. I had to provide for another life.. and to do that I had to get mine together whether the group rejected me or not.
I think friends have a large influence on forming addictions and on treating them. Peer pressure can play a big role in what kinds of behaviors you engage in, and when the people you hang out with act like something is normal and okay it can encourage you to think the same thing. Most treatment programs for addiction will encourage you to change your environment in order to stay sober--it will help you avoid the triggers that make you want to use drugs or alcohol. It makes sense that it would be harder to stop a behavior if you surround yourself with friends who are doing it.
There's been a bunch of interesting research lately on the impact of social networks on addiction--it appears it is a "contagious" behavior, meaning that having friends who use drugs or alcohol makes you more likely to use as well. I've written about some of them in the addiction news section of the site:
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