Its because of a little something called refraction. This is when there is a change in direction of what we call a wave ( such as sonor waves or light) due to a change in its speed. Also, when one object has a different refractive index, meaning a different ``speed of light`` than another object it causes this refraction. So, since water has a different refreactive index then air, it changes the direction of the light basicaly ``bouncing`` of your face and onto the water which creates a reflection. You see the light waves which are changing direction creating an almost identical image to what is standing in from of it. All in all, water has a higher density then air so it causes the light to deviate. This deviation can also be proven when you put a spoon, for example, in a glass of water and the spoon seems detached or bent from the part thats in or out the water; it dosn`t look straight and the part under the water looks bigger and very weird. So voilà, the magic of physics and optics :P
