Why do we like dessert, but not extremely salty, sour, umami, or bitter foods alone?
There's also probably a bit of biology involved: sugars are calorie-dense, and thus good sources of calories when you're struggling to survive on what you can forage. Bitter foods are often dangerous or poisonous in some ways; sour foods are often unripe, and thus harder to digest, and umami is a fairly neutral indicator. Salt can be dehydrating in large quantities. I have heard that this explains why little children need no coaxing to eat sweets but rarely like bitter or astringent foods.