Yoga, and many other forms of physical exercise, for that matter, cause the release of endorphins, and studies show that a person's emotional state directly correlates with their physical state, so if you're exercising regularly in a way that you enjoy, you're much happier overall, which can cause your entire body to respond in kind. Yoga is also very emotionally pleasing even putting aside that physical exercise gives a good effect, because it often includes pleasing stimuli-- candles, incense, comfortable clothing, a gentle atmosphere, sometimes even calm and soothing music. While it does have its benefits, however, it still cannot cure all diseases and can only improve the state of one suffering from emotional issues or certain bodily pains. It can't cure cancer, psychopathy, or even arthritis, though it can certainly improve the latter.