Unless all commodities share something in common regular exchange would be impossible. But as they all differ in shape, size, volume, colour, weight etc., it cannot be the physical quality or use value that allows them to be measured by a common standard. It certainly does not. So what determines the general price, the average price when, over time, all the fluctuations of the market are taken into account. Marx, like the bourgeois political economists before him, concluded that the key factor is human labour.
All commodities possess a definite quantity of human labour.
In earlier societies it was obvious that the exchange of products or labour was governed by a rough calculation of working-hours. This transparent operation of a system of values based on quantities of labour depended upon small communities, relatively similar forms of labour and the existence of an authority to balance society's books. But under capitalism this is impossible. If one firm can produce a commodity to the same standards while undercutting the labour time involved in making it, they will gain a short term advantage over the others. Over a longer period, the innovations involved in making that time-cut will reduce the average amount of socially necessary labour time to make both this commodity and all commodities.