Both of you are right actually and each gambler differs so basically we are seeing those gamblers that don't have control for themselves that they won't earn much money after they've just won in gambling.
But in real life, we don't know if they're the same person or there's already the life attachment for that daily gambler's life and then with the set of reality that he's dealing with and if he's emotional as well.
Anyway, it's true that someone's control over his emotion doesn't guarantee success in gambling but it helps right.
Emotional control is one great thing a gambler can make sure to get because if they can control their emotions, they will most definitely get to gamble better and responsibly as their gambling habit then grows to get a better gambling habit because they no longer just gamble randomly as their habit bis now under control as they have control of their emotions.
Success in gambling has nothing really much to do with your emotions but then it's a factor that can't be really over looked because after a few losses then your emotions could actually set in and this also applies to wins too because you will get overly excited or probably emotional about gambling and then will want to stake some more because it feels like you are on a winning streak.