One thing to remember about Nathan is simply that he’s OLD.
He’s open minded, smart, mentally stable etc., but (if I remember correctly) is over 100 years old. And he’s human. Yes, he has LV but since he doesn’t seem to have any LV-related abilities or weaknesses, his slow aging has to be related to his magical connection to his wereverine partner.
It’s safe to assume that the vamps and weres who survived longer than an average human lifetime are more mentally flexible than normal humans and given the rate of virus evolution, this would be a factor of natural (supernatural) selection.
But Nathan is just human. As far as he has come he also has a century-plus off habits and attitudes as a core part of who he is.