Explaining romance as a whole is too big and broad of a question. A few particulars:
A problematic pairing is someone making rapid, unbounded intellectual progress with someone who isn’t. Other than that, there’s no particular need to upgrade to a better partner. If someone else is doing good work relevant to your career, you can collaborate non-romantically (which is the more common type of collaboration).
There can be a lot of valuable knowledge in past history with someone, though sometimes there isn’t much.
There’s room for lots of differences between great people. Even if Rand and Popper both learned CF and fixed all their errors that I know about, they’d still be very different people than each other or than me.
Rand is weird about romance.
Romance tradition includes both reasonable ideas and bad ideas. A lot of examples from TV and movies are pretty bad (Friends, Sex and the City, ~every sit com or rom com).
Nuclear family tradition is reasonably OK.