Sugars act on the pleasure centers of the brain, like lots of other things and activities. Things that taste sweet in nature abound. As a kid, I learned to pull out the red flowers of clover plants and suck on the white tips where the sweet nectar is. As mentioned, obviously fruit can be very sweet, and also provide a lot of nutrients. Many veggies and even grasses are sweet to the taste. The "bad" part comes in when it's packed into everything and we become emotionally adapted to expect it in nearly everything, and then more recently in history, to where the sugars are refined so that they no longer behave in the body the way natural sugars do.
Aside from all of that, I think humans in developed nations are so far removed from survival and appreciating food due to its abundance (vs. the paucity at times that would have plagued our ancestors and does still plague our relative in the third wold nations) that we seek to indulge ("let them eat cake") because we can. Self control seems to have gone out the window in the last few decades, perhaps because it's not necessary for basic survival (note: "survival" in this context does not equal optimum health, feeling good and energetic, avoiding disease, etc, but simply being alive and functioning to some degree).
Add to all of that the fact that Big Agribiz wants to make money by promoting foods derived from the easiest grown and processed crops ----> corn into HFCs, wheat into everything, sell, sell, sell. Get us hooked and sell, sell, sell.