Romans 13:8-14 calls us to love others, to work for their souls' good rather than looking to please ourselves.
Why risk harm to your own heart or to that of a brother or sister to have a type of companionship that, outside of marriage, is arguably questionable anyway?
This brings me to my second argument against intimate one-on-one friendships between brothers and sisters in Christ. Men and women who are not called to long-term singleness and celibacy have a strong desire for companionship with a member of the opposite sex. As I've discussed before, Scripture seems to consider marriage (and children) to be a normal part of the progression toward biblical manhood and womanhood (see, among others, Genesis -28; -24; Matthew -41; Luke -36).
Basically, the question seems to be how exactly single Christians should relate to members of the opposite sex in that large and awkward zone between "we've never met" and a deliberate dating or courting relationship. I won't repeat the full history lesson here, as several Boundless authors have already discussed it (Joshua Rogers most recently, in his excellent piece "Your Friendgirl Deserves Better").
Essentially, the historical reality is that until 30 or 40 years ago, long, intimate friendships between men and women in which each served as the other's emotional confidante, relationship adviser and "best buddy" were far less common than they are today.
Yet even with all this deep communication going on, at least one aspect of these friendships inherently involves a mixed message.