Why does the price of a security (or future, etc) rise? Because there are more buyers than sellers? There can't be. For every buyer there is a seller. Because buyers are buying "faster" or more fervently? Again, that can't be because someone is taking the other side of those trades just as quickly. Is it because they buyer puts the buy order in before the seller picks it up and that creates a shortage? That seems to be the only explanation but in this day of lightning fast fills is that really quantifiable. It seems like an endless tug of war that should result in constant equilibrium. It seems so simple yet I'd love an explanation.