I have this theory that regular shampooing accelerates hair loss. Not only does the shampoo clean your hair too well by robbing it of oils, but when you rub your hair all over with your hands, more of it goes down the shower drain. It's like using a pressure washer to water your lawn.
Naturally, there's no financial incentive for anyone to study this (it's more profitable to sell both shampoo and rogaine than nothing at all), but I'd bet my scalp on it.

I decided I'd only shampoo twice a month, max. Water cleans it well enough. As long as I keep my hair short, it's never 'greasy'.