How to Change from BFF to romance?

Hello. So i wanna have a relationship with my friend’s account. However, first time i met him in a party i click add to contacts and somehow it just automatically connected as a friend. I wanna make it a relationship. Now I’m already on level 4 as BFF. i tried doing flirty conversation but still nothing because no event in that flirty section. So is it possible for me to turn it into relationship? How?