On a very small scale, it's certainly true that if I gain, somebody else might lose. If I take away my brother's chocolate, then he will lose it, and will most probably not get anything comparable.
But on greater scale, say, nationally, if one person (e.g. successful start-up founder) makes a fortune, will this generally be bad for the other players? Or can it be beneficial (e.g. if the money is not saved up)? Does it entirely depend on the rich person's spending behavior?