Oh how we wish it were just that simple, but when emotions get involved it's not. And we don't speak the same language. When a girl starts things by saying I want to be friends (at least I know when I say it) it means I want some time to get to know you without expectation or pressure that comes with dating. It doesn't mean that the door is closed forever unless that is very unequivocally stated, it just means I want some time to get to know the real you before I feel comfortable investing in the relationship at a dating level. I've never heard of a guy having this kind of thought process, I'm under the impression when a guy says he wants to be friends he either means he's not the least bit interested in you as a potential romantic partner, or he's not the least bit interested in any sort of commitment that would limit him in his freedom to do as he pleases (but might be interested in fooling around as friends with benefits).
For some of us, the idea that you will know when someone else is interested has proven to be completely untrue. Some of us have personalities that can't take a hint and if we're really blessed we also tend to be so reserved that it's easy to miss the subtle differences in our behavior toward you that mark growing interest (because there's so much difference in how I stare at someone in analytical interest vs how I normally stare at people in analytical categorization). Add to that the grasping at straws and or self doubt that comes over most of us when we're dealing with infatuation (either having someone tell us that the odds are a million to one that they'd ever go out with us, and hearing in that, that we still have a slim chance (a la Dumb and dumber), or seeing all the signs and dismissing them because we know it's silly to think that someone so amazing would like us and we're just seeing that because that's what we want to be true (a la what I've probably done to some poor guy and not realized it). Bottom line (at least for those of us for whom "you'll just know" has never worked): If you haven't directly asked, you don't know you only suspect (also other person might be waiting for you to ask so they know it's ok with you to express their feelings). And for Pete's sake, if someone has the courage to ask you a direct question, give them as direct and honest an answer as possible (preferably without using impossible to define terms such as in love which really don't communicate anything about your expectations for a relationship, just that you are experiencing intense emotions).
And all those of you gasping in horror at how cold and analytical I sound about the topic, now understand why I'm single (and quite possibly picky too).