even without the carbonation it still has the syrup to give it the good taste but for some reason that carbonation really adds some good flavor…
You know, that’s a darn good question. I would say it has something to do with the feel in your mouth, but I think water “with gas” makes me want to barf. Also, grape kool-aid tastes (a little) better to me than does grape soda. But I do love me some Pepsi, and it does taste nasty flat.
Carbonation makes the drink more acidic.
Carbon dioxide in water makes carbonic acid (NOT carbolic acid, which is a poison). This acid makes the drink more tart. I’m sure most people have noticed that plain soda water has a slightly acidic taste.
I would also guess that the “soda scientists” creating the syrup recipe tweak it to taste it’s best WHEN carbonated. So if the drink only tastes good when it’s carbonated, it was meant to be that way - playing off the “tartness” of the carbonation.
Perahaps another reason is that the gas comes out of solution on your toung (and other surfaces) so you are not getting the full strength of the soda as there is a gas barrior.
Carbonation does not improve all sodas. When we had a sodastream machine in ancient days it came with somepretty ropy soda flavours. I could drink the diluted flavours before carbonation, but they tasted foul with bubbles.