Hardest Pairing To Write?

Has anyone else noticed that while some pairings come very easily, others don't? Some personalities clash so much that it makes romance hard...what is the most difficult pairing you have ever written? And why? I guess this could go for original characters too, but I don't know why you would invent people that don't mesh well...

Anyway, the hardest pairing I ever tried to pull off (and failed miserably, I might add) was a HieixKuwabara fic that my friend requested...I mean, those two just...fight all the time, and not in a way that could turn towards romance. Plus, Kuwabara is dumber than a box of rocks in my opinion, and it is hard for me to write decent inner monologues and witty enough banter to keep the reader interested with characters who aren't exactly the brightest bulbs in the box.

Kuwabara does his best though. From what I remember in the show, what he lacks in brains he makes up for in loyalty. Since Kuwabara isn't much of a wordy guy, maybe you could express what he feels through his actions...
For me, some of my original characters I try to nudge together, but if their personalities clash, I just stop trying. 'Okay, they didn't work out- the show must go on!'
Anyways, I would have to say that the most difficult pairing I have ever written is my main character and his love interest. This isn't because they lack chemistry (trust me, there's chemistry) but because it took them so long for their relationship to develop. My main character had issues trusting men, while his love interest was raised in a strict society that have really harsh penalties for homosexual behavior. Trust me, as fun as it was writing their interactions, their relationship was A LOT of work... but it was worth it.
My main point is, if your heart isn't into the pairing, then you've already lost half the battle. If you find a way to write the story that you will enjoy it, then the readers will to.

To me, any pairing, whether it be original or fanfiction, that has the same kind of personality is hard to write. Yeah, they have similar qualities, so the romance should come easy, but it's hard to keep the readers interested. It becomes predictable, especially when they agree to each other on everything. To be honest I would rather write about a couple that argues about everything, it at least makes it interesting.