Intense interest in certain areas/concepts/periods...
(Mine has been *Ancient Egypt* for a few months now.) 
I am pretty sure there's an existential aspect to it where one sees the impermanence and fleetingness of everything, and then holds on to the concept invented in one's mind. I thought it is a sign of autism, but I am not sure now. 
It is culturally odd because normally the things one would hold onto would be things like family, friends, vacations, and so on.
Can being stuck on interests be a sign of a personality disorder? If yes, which one?