I get "obsessed", I suppose, over subjects which interest me. I am hesitant to use the adjective though, looking back, there have been many instances in which I did nothing except research and engage in these interests. I don't feel that is at all abnormal, however. (Alright, sometimes I tend to become completely enveloped.)
Specifically, I am interested in languages-similarities, differences, evolution, grammar, idioms-and psychology. I suppose I'm interested in the latter because, lacking in certain qualities, human interactions and behavior have always been fascinating to me. If it weren't for my interest in human behavior I would not have learnt how to socialize.