Being nerdy. Seriously, that's my honest answer. It doesn't even have to be an interest I'm interested in myself, but people who dare to be unapologetically nerdy about something just gets me really interested.
(Exceptions does apply. See: Furries and such)
Maybe it's because I was young when grunge was a thing and to be cool, you just had to be that kind of "Don't care about anything, ever"-type. Barf. So boring.
One of my exes had a fancy degree in a field (Soft sciences) I just plain don't respect. I mocked her. She held her own. Schooled me on its importance and beamed with nerdy interest when she spoke. I fell for her, and hard. And that's the story of why Swagstika is also into girls, sometimes, for the right person.