Why is that the first thing they change?

Why is that when ever newer model cars come out, the first thing they change is the headlights? I mean everything else on the car looks pretty much the same except the headlights. For example, a 2002 PT Cruiser looks the same as a 2008 model except a slight difference in the shape of the headlights. Another thing I noticed are cars that look the same but has a different name and manufacturer. Like GMC Sierra trucks look just like Chevy Silverados, Ford Fusion looks like a Mercury Milan. Mercury Mariner looks like a Ford Escape. I can't say for sure but everytime I see the Nissan 350 Z sportscars, I can't help but see a slightness of a Porsche 911. I first looked at the body of a 350 Z and that of a Porsche, especially the back and side view of both cars and they look almost alike to me in some way.