Riding against traffic sucks.

First all the cars pulling out of driveways and such are not expecting you.

Secondly the speed difference between you and traffic is much greater. Let's say you're riding 15 mph and the cars are going 35. Riding with traffic the difference between you and the cars is 20 mph, riding against the difference is 50 mph. And that means not only are you passing cars at a worse relative speed (riding with traffic a car only needs to reduce speed by 15 mph to avoid hitting you, whereas riding against traffic you both have to stop completely or one of you needs to turn aside to avoid a collision), but you're also passing more than twice the number of cars.