people already think this as bad because it can make people who already worst to become more worst. It can make people problematic the way they are going to think where they get their bankroll on next day. It can make person addicted on it which it lead them in bad situation.
Greed, wrong mentality and over indulgence, is one thing that has always been a characteristics of most gamblers and the main reason why they have ended up an addict over time. Being an addict is very bad in anything because it has fully taken control over you and it is more like you worshiping it and not being able to live a day without it.
For that fact, it makes them poorer as they make wrong decisions and judgments and they keep losing so much even though it is affecting the people around them. This alone is enough to bring about the negativity that surrounds it.
What has always been the major problem when it comes to gambling is that they focus too much on winning and discarding the reality that this is just a game of luck and the chances of winning is usually very slim. However, before they would start realizing that, they would have been long deep in it and getting out would be a problem and we all know what the end result is usually like.