Because it tastes sweet which changes your palette to prefer sweeter things generally. Some of these will likely contain sugar. So you’ll eat more sugar than if you drank water.

Because your palate gets saturated by sweet and as such many things that have a low sugar content are not appealing to you anymore. For example, when I was younger I was the only one in the family who drank soft drinks. Every time my mother made an apple pie it almost tasted sour to my mouth even though the rest of the family found it very sweet and pleasant. Wouldn't you know it, when I stopped with the sweet soft drinks my palate adjusted and I could finally enjoy my mother's low sugar pie