Why did Doctor say stop smoking weed while giving me a Ambien prescription?

Personally, I think disapproval of weed is down to the fact that it is illegal, so people automatically attach a stigma to it. But....realistically i think it can be less damaging than alcohol. I know that for me, I can go out on the town, get drunk and act like a total idiot. If i sit at home and smoke a spliff, i just feel relaxed!! I understand that the manmade chemicals in weed can be damaging, but so is alcohol and tobacco, and if they were discovered now instead of centuries ago, they would be illegal too. If the meds from the doctor make you feel uneasy, I wouldnt bother taking them. They may be just as damaging!