I can kinda get that, but then again, I kinda don't. I don't see much challenge in it, especially when most pairing fics usually go the route of "Hey, I suddenly have feelings for you!" In other words, they fall back on natural attraction and hormones. I don't see many fics where the love is one-sided, and one person has to work to get the love of the other. They usually fall for each other at about the same time. As a result, it doesn't feel natural to me.
I just realized that sounds a bit contradictory. Let's just say that, for me, when I read most "romance" in fics, there doesn't seem to be much challenge in how they are paired up. Just "oh, I suddenly like you. And I suddenly like you, too!" And then most of what is written are dates or them making out. I haven't seen the "why" being demonstrated, and that's the real challenge. So, for me, romance has to come about naturally as a result of the fic, and not feel forced. Circumstances throw two people together, and over time, because of shared interests or shared circumstances, feelings develop.
And let me say, I have never seen the opposite of this thread... two characters growing apart because of irreconcilable differences. Perhaps a fic where two people get together, but the feelings they initially had weren't enough to sustain them. That was partly my reasoning behind Fate/Yuuno in Future Tense. They got together for the wrong reasons, and as a result, they grew apart and separated.
That, I think, is the true challenge.