You are right that if a player knows his limits, they would never have a problem with gambling. Excess of almost anything is a problem, be it gambling or alcohol or just anything really.
Anything done that crosses the line always seem to be considered bad behavior that end up harming. Gambling must also have control so that gamblers can prevent many of the problem that exist in gambling, including finance. I have no serious gambling addiction and I have no habit of drinking alcohol or smoking marijuana. This allows me to control the gambling and so far I'm doing well without too many problem.
Being greedy allows the house to win more, and if you are not responsible gambler you’ll ended up on a bad situation. I also don’t have any addiction, because I control my gambling activities and I rarely drink alcohol and never smoke anything, this is indeed a big help. If you’re into gambling right now, make sure to know when to stop, don’t let yourself ended up broke because it will not be easy for you, just be simple gambler who plays for fun and not for the sole purpose of money.