Milk ‘sours’, ‘goes bad’ or ‘makes yogurt’ due to bacterial action. The key is what kind of bacteria get the upper hand during these processes and end up in the majority. When you make yogurt you are creating a milk product with a chosen bacteria population. When plain milk sits in your fridge or on the shelf at the store any spoilage causing bacteria that is in it will begin to multiply. Pasteurization only kills a certain percentage of bacteria. Yogurt is milk that has been pasteurized to kill off most of the un-wanted bacteria, then inoculated with beneficial bacteria and allowed to incubate at a bacteria friendly temperature. The beneficial bacteria take over and fill all available living room and use up the food supply so even if alien bacteria (spoilage causing) were introduced they could not survive. Traditional yogurt also has a high acid content, which many bacteria cannot survive in.