Weeds vs Grass

Why is it that weeds will grow on a friggin rock but grass won't grow even when you water it and tend to it??? It's a rhetorical question so please don't send me the scientific answer. Last evening around 8:30 it actually cooled down alittle and there was a slight breeze blowing. I forced myself to go out and pull weeds. At first the weeds were just pissing me off but the longer I worked the more chilled out I felt. It was kinda ZEN. And when I was done I felt superior to the weeds.