The single main reason we lose at gambling is our lack of self-control. We cannot stop ourselves from gambling and gamble constantly and sometimes panic and gamble. Due to which our losses in gambling are more than our gains. And this addiction takes a more dangerous form when it comes to gambling. Which destroys a person's life instantly
That is always the main problem. You can be disciplined and say that you will stop after you lost x amount, but some days (and these days will come), you will keep going and playing riskier bets to get your loss back. In that moments, you can loose your entire bankroll but that is only something that you realize afterwards.