I'm just interested in the psychology behind it. Despite all of the advice and logic to avoiding it, I fell into the Oneitus trap in the past few months. And only just started recovering from it...

Which leads me to think from my own experience, that Oneitus is actually built into men's psyche. I had no control over it.

I explicitly told her I only wanted a casual FWB relationship, but failed to keep it that way. And then she jumped ship for a colleague at the gym she works at.