Specifically, this is where a person has enough positive attributes to be considered a friend; but without the spark that goes with a true romantic relationship. In other words, he or she is stuck in a Platonic relationship. (Poor Plato; he must have felt rejected.)
This can indeed be a lonely space. In part because in effect one's genes are deemed not sufficient for being passed down to other generations. Or, to use a sports metaphor, not being selected for the playoffs.
It tends to be unwelcome news; except with the "friends with benefits" possibility. Some crass people might see that as a free ride without any heavy emotional lifting; but they are congenital freeloaders. For them, free sex without emotional burdens is what they want.
However, because some guys have low abilities to receive interpersonal cues, sometimes they have to be explicitly notified. Hopefully, not in such an extreme form as this notification:
We've all been there at some time or another. But here is a major truth to remember: