personally, because it can destroy lives of both the user and the anyone close to them. I have seen it first hand, a friend used to deal weed and every time I was there I would have to listen to customers talk about nothing else except drugs. They were always falling out with other friends and family, always over money or drugs. This lads girlfriend is a user and not long ago smashed up her third council flat; she's now banned from the local coop for stealing, but still goes in and steals, because the police say there's nothing they can do because she always wears a hoodie and they can't prove it's her.
But this actually is a good reason to have decriminalisation; if it was cheaper, easier and safer to get and take it, then maybe they wouldn't have to steal. But I also think if someone has problems, giving them a fix might not be enough. The whole sorry state of addiction is that sometimes you can never get enough. It's at this point I'd worry what would then happen, if they'd had their "legal" quota for the day, would they just go out and beg/steal/sell their body to get more illegally?
"because it can destroy lives of both the user and the anyone close to them". But I suppose those close to drug users might not have a problem if it was done safely. We tend to assume that drug addicts will end up dead and they're wrecking their life, but who knows, perhaps in a safe and controlled environment they might not. Probably trials need to be done on a small and impeccably managed scale to see what the results are and if it could work on the wider population?