Why when I start my car do I have to reprogram settings on my new car stereo?

Other answer:

Fiona:
On the back of the sterio there will be a wire that needs to be connected to constant power to keep memory alive…I belve it is yellow… good luck…
AL M:
Why, because the constant positive was not wired properly. It was most likely wired via 12volt positive via ignition switch.
derek:
make sure all the colors from the wiring harness too the head unit are wired up too match. Red-red, yellow-yellow etc.
N2Audio:
not if it's installed correctly.