Yogurt can be incredibly healthy. Unfortunately, most yogurts found in the grocery store are extremely bad for you. They are frequently low in fat, but loaded with sugar to make up for the lack of taste that the fats provided.
Put simply, the yogurt has had the healthy, natural dairy fats removed, only to be replaced with something much, much worse.
Additionally, many yogurts don’t actually contain probiotic bacteria, as generally believed. They have often been pasteurized after fermentation, which kills all the bacteria.
Alternatives: Choose regular, full-fat yogurt that contains live or active cultures (probiotics). If you can get your hands on it, choose yogurt from grass-fed cows.