What I remember best from Kahlil Gibran's "The Prophet" comes from his chapter on friendship. "Let your best be for your friend," he writes. That has stuck in my mind since, living single, friendships may occupy a more essential role to me than to the partnered guy.
There's something troubling about the use of the word "relationship" that excludes all relationships but romantic ones. All other adult relationships aren't just excluded in the wording; they're absent from the studies.