Some things strike me as just being plain “not fair.”
Here in the states we have one kind of Schweppes Tonic Water for sale. Its pretty much exactly what you expect: saccharine, sweetened with high fructose corn syrup, and overall pretty much indistinguishable from other plastic bottle tonic waters available in supermarkets across America.
But overseas, there is this other kind of tonic water called “Indian Tonic Water.” Which (spoiler) means the exact same thing. It is simply put, tonic water. However, taste-wise it is radically different from anything we have on the mainstream US market.
It is slightly sweet, with a syrupy undertone, cut cleanly by a brisk dose of quinine that tastes at least two or three times more intense than the Schweppes I usually buy. The less-intense sweetness means that when you mix if it with gin, a lot more of the gin flavor actually comes through. Overall though, it makes for a much more bitter cocktail.
This is the kind of tonic that really made the lemon/lime a requirement. With not a lot of sweet, it leaves room for the natural sugars of a citrus fruit to make themselves known; whereas with your usual tonic water you might not even need that.