@Koni: I don't think that's the only thing, either. Though it's related to what you said, I still think it counts: on top of the good pace the romance proceeds at, there's no suspension of disbelief when it comes to the girls. Eruko is genuinely gorgeous, you don't have to force yourself to believe that she is when the author tells you so. You never catch Hoshi stalling or losing certainty with Eruko because some girl who's obviously not as attractive (in whatever way) came along.
Romcoms have become infamous for BS, by now--this story has none.