Why do my brake lights turn on when the headlights are turned on?

If your brake lights are always turned on whenever your headlights are on, how will the driver behind you know you will be stopping? The state inspection people have been remiss with your car, I think. The reason why brake lights have two filaments is that it is supposed to turn brighter when you apply the brakes. If they are always on, then the second filament is always lighted up so no more properly functioning brake lights for you. A good and patient electrician will know the trouble. Get it fixed. You have been lucky you haven't been rear-ended for 4 years. Your luck might just run out.