I think ego drives it all for me. If it's ridiculously horrible weather I love riding in it because it makes me look so tough and die-hard. The more people who see me the better. But after a few days of that, we have a nice day-- and that's when I take the car!
As far as which route I choose, it's based on traffic. But apparently higher traffic roads that are well within my comfort zone are outside of many people's. I'm frequently astonished that people aren't comfortable on streets I use daily.
2009 Trek 7.2FX WSD, brooks Champion Flyer S, commuter bike