Or "Why didn't anyone explain all of this madness to me when I began?"
Use CSS sparingly to control a few page elements, and you'd be right to call it a simple methodology. Start pushing the envelope and begin working across multiple style sheets with all sorts of conflicting selectors, and all hell can break loose.
The thing is, this "hell" is intentional. Well, not intended to be hell as such, but it is a system designed to flex its muscles and produce results more powerful than anyone could imagine when they begin to toy with CSS.
CSS cascades, it inherits, it can be grouped and contextualized. As if that were not enough, there are numerous units of measurement to be considered before diving in, each relevant ...