This isn't a rabbit, I'm just relaxed...

According to a program I saw on Discovery Channel, male arousal happens when nitric oxide is sent from the brain to "the area". The musculature of the penis is spongy, as opposed to the more fibrous nature of other muscles. When flacid, the penis' muscle is actually taught, restricting blood flow. The nitric causes the muscle to relax allowing free blood flow and the resulting expansion.

There are those with additional letters behind their names who hypothesize that female physiology works similarly.

Viagara actually works by maximizing the effect of nitric oxide, though there are concerns over it.

Since it is basically an issue of blood flow, the same issue can be addressed with vitamins/exercise.

Basically, you'll be happier if you live healthier. Yeah. Big news.