Just to pour in some first hand experience, I recently married my best friend whom i've known for almost 7 yrs now. Its so much fun being in a relationship which started as pure friendship! As we've always been together as friends, it was a roller coster ride for us to dvelve into our emotional dependencies, But once we discovered that both of us have secretly passed on that thing called love into each other's lives, it was an obvious & most comfortable choice to get married!!
So, the point being- If u know what u r into & enjoy being there, why not go deeper!!!!!