I was explaining this to my daughter in quite simplified terms the other day- we evolved to taste sugar and enjoy it because finding a sweet edible plant meant we had a source of energy to help us hunt that day. Pretty useful if you're a hunter-gatherer.
So we seek out sugar. Now we can get it whenever we want it, in much more massive quantities than we are supposed to be processing. Most of us are addicted. I'm not an exception.
Fruit yoghurt is pretty much yoghurt with fruit jam added, so it ends up with quite a lot more sugar than the natural stuff which has no added sugar, so ever since I've had to start watching out for my sugar intake I've started only eating the natural one and adding cinnamon or vanilla extract for flavour.
It's amazing how after a while of cutting sugars from your food you get used to it, don't feel the need for it anymore and even start finding the most sugary stuff (like certain kinds of sweets) unpleasantly sweet.