hmmm theres 2 reason why guys and girl become good friends
1.guy/ girl want court the guy/girl
2.just like to be/feel comfortable when with them,and they can share problem with each other,some thing like sis and bro

i my self also got girl as good friends,i treat them like my own sis,and nothing more,and they are very close to me,i help them out when they need help