From time to time we come across various gambling related information where bank roll is mentioned i.e. you deposit a certain amount of money and conduct gambling. If this is not done, many times a gambler has to face various problems. One of them is that placing the wrong bet can lead to huge losses. Loss is very common in gambling but if you are addicted there will be disaster. Nothing excess is good. A gambler must learn to control himself before becoming addicted.
A person shouldn't learn to control himself before he gets addicted, but he should learn to control himself before he starts gambling. Having control over your emotions will be useful in any field of life you have, and in gambling, it plays the role of a savior as it saves you from getting excessive losses due to mixed emotions and trying to recover losses.
Addiction to anything can be a disaster for a person, even if the thing isn't bad itself but if you get addicted to it, that thing will start proving bad for you since you cross all the limits for that thing and as you said, excess of anything is wrong.