When I study the forward contract, I read that the forward price must be the price that makes the the value of the contract zero.
I searched for the answer, but there are many versions.
Some say it is because the assumption that the information is available to all, so the buyers and the sellers must be indifferent of whether long or short the contract.
Some say it is because of the no-arbitrage principle. But I still cannot fully understand it.
Why cannot two people sign a forward contract, one pays another some money while setting the forward price?