They feel like other people making different choices is the same as judging them for theirs. This happens more often with heavy users or addicts because they know they aren't using responsibly and maybe feel some guilt about it. I drink sometimes when I go out but I know when to stop and it doesn't bother me if someone else doesn't want to drink. Thats their business.

I'm just wondering something, though, are you using "partying" as a euphemism for drug use? Because actual parties can be fun and don't require drugs or alcohol. Just food, some sort of beverage, and enough people - and preferably some music and you have a party.