Why should you ever be a guesser? Because guessers exist, and sometimes it's not in your interest to offend them.

I think what makes people tend towards guessing is a combination of personal temperament, childhood upbringing, and current environment. If you're sensitive to criticism and rejection, and you're good at picking up emotional cues (or at least if you tend to read emotional states into people; you may be wrong about those states), then you'll really want to avoid upsetting people with your requests. If you were brought up to believe asking is rude, you won't ask much.