Keeping the temperature stable around 40C helps the bacteria you want and keeps a healthy yoghurt culture. Too much hotter and the bacteria will die- too much colder and other bacteria will take over. I have made yoghurt without a thermostat using a thermos flask and starting it at about 43 degrees C then wrapping the thermos in a blanket and putting it in the sun - in hot weather (28C plus) this has worked as it keeps the yoghurt culture at a stable enough temperature long enough. It takes 5-6 hours for the yoghurt culture to convert the milk to yoghurt. A thermostat makes it easier and more consistent. Given the price of store bought live yoghurt you can save a lot by making your own.