It's the general feeling that matches, I don't think neither music nor human emotions can neatly be packed into boxes.

Nobody is ever feeling just in love.

Or a romantic song could just sound optimistic and hopeful to you and you're optimistic and hopeful in that moment.

And of course what sounds romantic to one person might not to other.

It's not clear-cut, it's just a general trend they noticed.

But also you could have read a romantic book, and be in such mood even though it might not have happened to you personally.