Maybe you like the idea of being the 'dominant' partner. Sometimes passive people can be quite attractive and perhaps it's that mystery of their true character that you find interesting?
Also, it's probably quite rewarding to be able to 'discover' what someone's like, instead of dating someone whose personality is more open.
Also, it's probably quite rewarding to be able to 'discover' what someone's like, instead of dating someone whose personality is more open.