If we let curd for more days, the yogurt become more and more sour why ?
Lactic acid is produced by 'probiotic' bacteria breaking lactose into lactic acid. Over time, more lactose is converted, producing more byproducts, thus more sourness.
The byproducts of this reaction are responsible for the distinctive flavor of yogurt.
See a more complete description here: Lactic Acid Fermentation