it has to do with your speed and surface area relative to the rain. the faster you go, the more you are "running into" rain drops as opposed to having them "fall down" on you. it tends to be similar for most walking and running speeds. the big difference is all of the muddy water you kick up onto your pants and shirt if you run instead of walk. but sometimes its fun to run and dodge individual drops.

personally i enjoy riding in downpours.