Is plain yogurt supposed to be sour?!

Is plain yogurt supposed to be sour?

Or did this yogurt go bad? I'm using it in a recipe and I wasn't expecting it to be sour. I'm using a ton of sugar it isn't really healthy anymore.

As per our Indian standards, plain yogurt is not supposed to be sour. Normally we get the yogurt set at home, so we take care to add the correct quantity of yogurt/ curds to the lukewarm milk. This will be kept to set in warm temperature for around 6 hrs. It turns either sour or tart depending on the hrs it stayed out after getting set. So we normally allow it to set and refrigerate it. Commercially, they add sugar or essence to mask the oldness of the yogurt.