Gambling portrayed negatively because of the main reason gambling can have a negative effect to the person involved. And also I think gambling can lead a person to be addicted,greedy and some will commit crimes just to get a money to feed their addiction.
Gambling addiction is same like drugs/smoke addiction though and the result is always negative, i'm sure there are many lifes that become ruined because of gambling and also gambling is a sin on all religions. Even in my country, gambling is banned here.
I think it is bigger addiction than that of drugs because it can happen that the drugs supply discontinues due to some reasons but these gambling casinos will never be stopped or banned in the country where gambling has more followers because these casinos are paying huge amount of taxes to the governments as well as other amounts.