Because love is only a compromise, in the end.
Sharing
Sanji shares Zoro. First of all with his swords, which he spends hours on end polishing and practicing with and just staring at, memorizing them. Even when he sleeps, they are next to him, on handle gripped in his fist just in case they are attacked.
He shares Zoro with the girl whose name he never heard, who he knows is always in his mind, and the man whose name and face he can't forget, even though he'd seen him for only an instant. He shares Zoro with his scars and his determination and his training, with his need to be stronger and his faceless, nameless drive to win a title that may not even exist.
Zoro shares Sanji. If asked, he would say he shares Sanji with his kitchen, the place where he is found three quarters of the day. With his ingredients and recipes, most of which Zoro couldn't follow and few of which he even understands. He shares Sanji with Nami and Vivi and Robin and a thousand other girls who he is in love with, and not the fake, faceless kind of love, either. Because Sanji loves completely and he loves always, and if Vivi is on an island a thousand miles away she is still here for Sanji, just like every other girl he's ever loved; will always be here, in his eyes and in his voice and in his smile. Zoro shares Sanji with the Baratie, with All Blue, that mystic place he may never see which to Sanji is there, always, right beneath the skin.
So of course they end up fighting, and Zoro has to occasionally punch Sanji till he stops simpering already and does something productive (which starts out being dinner, but ends up more like sex on the galley table). And Sanji does end up kicking Zoro in the back of the head at that exact moment when he knows Zoro is just about to fall asleep for his afternoon nap. Usually he wants Zoro to help with the dishes, or some other girly thing the swordsman can't appreciate but does anyway just because he's so damned tired of Sanji sometimes he'll do anything to make the stupid cook happy.
Sometimes, though, they have to stop.
It is an uncomfortable thing, to realize that Sanji really is too busy for sex, because he's got bread baking in the oven and half a dozen dishes in various states of preparation and dinner's so damned close if he doesn't finish there won't be anything left after Luffy's gorged himself again. Zoro always feels out of place, realizing that there's a difference between Sanji being a tease and Sanji telling him to fuck off entirely. He wonders out onto the deck again and sulks until dinner, stubbornly practicing the same move over and over again and wishing, not for the first time, that he'd picked someone whom he could understand better.
Sanji feels the same way, of course, when Zoro really isn't going to help with the dishes, because in the end he can't understand what is so goddamned important about standing there in the freezing Grand Line weather wearing practically nothing and swinging the dumbbell over and over again, as if it would somehow mean the difference between life and death. Oh, he knows logically that to become the best you have to spend some time training, but really it seems more like a joke when Zoro does it, nothing to be taken half as seriously as helping Sanji out when he's tired after cooking all day. After Zoro starts ignoring him (and eventually simply swings the dumbbell at his head with a seriousness that would send Ussop screaming), Sanji always feels on edge, waiting for a confrontation that never comes, expecting some sort of retaliation. When nothing happens he starts to wonder and get anxious, which leads to getting angry, which leads to petty fights in the middle of the night when they're both so fucking tense around each other for such stupid reasons.
Sanji isn't stupid. He knows they don't really see eye to eye. He and Zoro are fundamentally different. And he hates that Zoro can't understand why All Blue's so damned important to him, why he prefers cooking a six course meal to screwing on the kitchen table, what he gets out of simpering at Nami-san and Robin-chan when they obviously don't care.
He hates that Zoro so often sees him as weak, the way Zoro makes a point of telling him over and over again that Nami-san is using him, is just taking advantage of his stupidity, as if he doesn't already know it, as if it's such a crime.
He hates Zoro's will, sometimes, even though it is really what he likes best, too. He hates how Zoro can't back down, can never just give it a break and give a little.
He hates the way they can't quite get close enough to be comfortable around each other.
Zoro notices, too. He realizes that Sanji isn't really a warrior, in the end. That being the best seems stupid to him. He knows that Sanji has no real ambition to become the best in anything, even cooking. He hates the stupid cook for being so content, for being so weak sometimes, for keeping up with Zoro just enough to be annoying but never enough to be someone Zoro can look to when he himself is down, like Luffy.
He hates the bastard for his stupid crushes, for being so damned polite to women, as if they're worth if, for letting them step all over him, for not minding being taken advantage of. He hates Sanji for being so easy, so damned free, as if anyone can come along and take a piece of him.
He hates Sanji for the way Zoro knows he could fight, because Zoro has watched him cook enough, handling knives with an annoying amount of expertise to realize that Sanji could be better than this, could work as hard and try as much and be more than a cook in a kitchen, always subservient.
He hates, most of all, the way Sanji is, despite it all, the only person he wants.
It's no wonder, in the end, that they fight more than they screw and they argue more than they talk. They are used to it, after all, and screaming at each other like children, throwing kicks and punches that could kill weaker men is easier, perhaps, than admitting that they cannot accept things as they are. Zoro will always keep pushing, striving, and Sanji will always be left behind to do what he loves best. Both of them are comfortable with their way of life, but in the end the act of compromise is hard, despite how much they want it. Sometimes Sanji wishes he could just meet a girl, because girls are easy to please, easy to love and understandable. Zoro in turn wishes he had never even gotten it into his head to mess with the cook in the first place. Everything had been so much easier before.
In the end, what they both hate most is just how easy all of this is to forget, when they're curled around each other in a shared hammock, so damned comfortable and familiar. After all, it's hard to share, but it's even harder to let go entirely.
END
A/N: Does this sound painful? I like to think that it does, and maybe this is a little too deep for One Piece… I really like the SZ couple, but in the end am too much for a realist to write them like I want to read them: uncomplicated, accepting, etc.