I always drive in the lines and am admittedly pretty 'particular' about it, but my wife just drives all over the place - I've said it time and time again and it frustrates the crap out of me, but what's a guy to do? I've done the argument thing, she is just not a great driver and thinks they're just general guidelines.
My theory is that I always drive like there's someone in the oncoming lane - how would I drive if there was oncoming traffic? I see it all the time; a quiet road, someone in front of me doesn't see any oncoming cars and goes over the lines, and when a car comes around the corner or over a hill they have to jerk back into the lane, coming dangerously close to causing an accident. They design the roads to be pretty easy to follow, just figure it out and keep everyone safe. If anything finding the apex staying inside the lines makes it more fun!