I have never understood why the US and India did not form a stronger trading relationship until the past decade. I think that India was reluctant to be "re-colonized" and so its foreign investment rules were an impediment to forming joint companies with American companies.
umm ... probably because India was giving the Soviets a naval base during the cold war??
and because untill 1992 Indian policies were protectionist and what the U.S. would call socialist and slightly communist even.
It was after 1992 when India almost went bankrupt that they opened up their markets and China became a counter to the U.S. that it seems India would make a good ally to counter China's influence.
Other than that, I dont get the "natural" part of the ally.