How many times have you tried to get the perfect mixture of hot/cold temperature before showering, only to have the hot water slow to a trickle just in time for you to hop in?
The reason behind this is simple. Cold pipes are smaller and expand when heated up. The thicker the pipe, the more pressure is required to push the water through at the same rate, which is why you need to turn up the hot water occasionally.
I wish some day that a smart person would invent a cheap material that would not expand when hot water flows through it, making it unnecessary for me to adjust the temperature!