My God! Why is this so good? Why is this so good?!
Well, if you start out with a naive but plucky cutie pie who is totally unaware of how adorable she is, then add in a beautiful mentor desperately trying to hide how totally she's fallen for the cutie pie, and make the cutie pie all but oblivious to how everyone, the mentor especially, adores her, and put everybody in sharp but cute uniforms, you've got yourself a pretty solid foundation to build on.
There's a lot more to the storytelling than that, of course. But there is that.