Smirks like a proper wizard.

Well I always assumed it would have more to do with the energies of the Aether. I mean mages would be more drawn to other mages because they are more on their level either of understanding the world as they see it as knowing, feeling and controlling the Aether.
For example to Tyrhallan it is simple fact the Aether is there because he read it in a book and he can see the effects of it. But he doesn't feel the Aether or link with it as much as Arn or Eilis does.

Whilst I cannot speak for Arn I know that Eilis solely depends on him. From the moment she awoke on the ship to now she considers Arn to be her lifeline. Without him she would be lost and more than likely dead or worse, so she regards him as her saviour, a true friend. Eilis has had some twisted upbringing she doesn't really know how to act or what society standards are, she simply is used to being told what to do.
All she knows is here is a man who is supposed to be my enemy and he offered me help and comfort forsaking his own...he has been nothing but welcoming, protective and kind and Eilis certainly feels connected to him.  

Being too Amorous though...I don't know. Maybe a tad, but I don't consider it troublesome. They are mages...mages are weird in the first place so maybe it could be simply the link to the Aether at play here.
Usually there is an underlying attraction whether it is physical, spiritual or simply a shared emotional moment it only becomes amorous if Arn truly seeks to pursue it.