I've seen a lot of 'Reinako' fan art lately, most of them explained that's because of how they are in the manga... But I haven't read the manga yet, I truly don't know if their relationship is different from the anime, I ship them anyways!! :B
^It's the manga and the live action drama, in which their relationship is a lot more developed and drowning in subtext. And also the love affair between Venus and Mars in mythology. (Cupid is their son in some accounts.)