I think one of your assumptions is flawed: That commuting by car can be non-painful in any condition.

Now don't get me wrong, there are times when the car is the best tool for the job. I am not anti-car. That said, for my particular car commute, it is never fun, just more painful some days than others. The only bicycle commutes that I have not outright enjoyed are the times I've crashed or extreme tired due to extreme wind or other factors. Here's some more research on the psychology of driving:
From Dr Leon James:
http://bicycling.com/blogs/roadright...-is-road-rage/