1. of or relating to formation, development, or growth: formative years
2. shaping; moulding: a formative experience
3. (of tissues and cells in certain parts of an organism) capable of growth and differentiation
4. functioning in the formation of derived, inflected, or compound words
Could this be related to the fact that cheese is "formed" through fermentation of milk?
i.e. leftover milk was fermented to not let it spoil (whence it's a product of necessity), so they had to eat it as cheese?