I think it's about money, when there is money involved it gives you adrenaline rush, and gamblers seems to be addicted to it and they can't seem to find that feeling anywhere else.
It's not about money. Money can be involved both in games, such as tournaments for example.
Gambling has other features that provide you pleasure playing. For example, trying to set up a strategy to fool the casino makes you think that you are superior over them, if you know what i mean.