I come from a more continental-climate-area in the US so I'm used to a range of temperatures from -15C to 35C. I just can't imagine wearing shorts under 15C unless I am exercising or indoors.
I live on a 17th century farm which is heated by a wood-burning stove and a wood-burning kitchen stove. While it is quite possible to make it too hot for comfort inside (record is 27°C inside with -20°C outside, the result of adding a bit too much wood to the fire while leaving the ash door open) the inside temperature usually lies somewhere between 16°C and 18°C, in the morning often around 12°C. Having a lower inside temperature makes for a smaller temperature difference between inside and outside. This in turn makes for less of a cold shock when going out.
Humans are, to a certain degree, more sensitive to temperature differences than to absolute temperature. If it is 17°C inside that 15°C outside can feel quite balmy. If it is 23°C inside those same 15°C feel quite chilly.