By Definition
It's not what you all think. Everything doesn't just "fall into place" when she's there. She's hitting him half the time and calling him names, mind you.
It's just that when she's not there, something is… off. That's not because he misses her – which he does, and he's not too scared to admit, he just doesn't bother to – it's because when Rukia's not with him, she's usually in trouble somewhere. He should know, he's always cleaning up behind her.
Somehow, he has yet to figure out the difference between her just leaving for Soul Society regularly and her leaving to do something immensely stupid. She seems to end up doing both all the time, which makes it really hard to tell. After all, it's not like he can smell her from afar.
Come to think of it, it makes him really uncomfortable that Orihime can do that. Sometimes he wonders if she can tell what he's doing just by testing the air. Does he work up a sweat in his spirit form? He's always running and fighting in it, and there's always the sickening, rusty smell of too much blood being shed. Not to mention he has a sneaking suspicion that his breath is not all roses either, he's fought enough Hollows to wonder.
He knows Orihime wouldn't mind. Might have something to do with that she loves him, but he absolutely doesn't want to go there. Talk about feeling uncomfortable.
You'd think he'd grown accustomed to the general idea of girls and boys (or maybe something else) – he's lost his mother, he's been through a war and he's seen things that other people… let's not talk about that now. Let's just say, if there's one thing, in this world or another, that he's one hundred percent sure of, it's that life can be short. But still, you're wrong: he's seventeen by now and the whole concept ist somehow foreign to him. To the point that he still stumbles over a word like "kiss".
He's not stupid, of course, he knows there's something inside him. It tends to get to his head when he's near Rukia, which makes it all the more frightening. It makes him lend her his jacket when she's cold, or buy her things when she's curious, or cross the earth – and then some – when she's in danger due to all those stupid things she does that were mentioned aforehand. But he doesn't have a name for it, and stubbornly refuses to pick one. It's certainly not… that, you know, it just isn't. Why are people always asking him that?
And while you're at it, how about not cracking jokes about him and Rukia in bed? Because he doesn't think about that. Ever.
He's learned that his teenage urges can manage themselves. The morning after he never remembers what triggered it, he just discreetly disposes of any evidence and instantly forgets about it. Instantly. Every time.
He hopes he can live through High School and College that way, because he thinks of those other things you can do as troublesome and rather gross.
For example, he's quite sure that Tatsuki would just gulp down a boy in the middle of kissing. He's seen a documentary about the praying mantis once, you know, those things that bite off their partner's head when they're… at it or something. He's pretty much relieved that he won't have to look out for her that way, she'll be fine. But sometimes, he catches himself watching her mouth in horrid fascination, trying to detect signs of fangs or maybe some leftover school uniform between her front teeth. Never fails to make him shudder.
It's even worse with Orihime. He harbours a strong fear she'd trap him between her breasts if they ever kissed. He suspects that she'd marry him on the spot and cook all sorts of awful food for him until the day he dies. Which wouldn't take long, because there is an army of children in his horror vision of their future, pulling his hair, messing with his work, slobbering all over the place. Now that is scary.
If you absolutely must know, he's tried kissing once with Rukia, and it wasn't half bad. He was on his way home from school that day, and it was freezing. It had been a tiresome day, he'd been completely unprepared for the biology test because someone had really grated on his nerves with their obstinate not being there the night before.
Rukia arrived exactly when it started to rain, and she seethed with anger. They stood completely soaked in a matter of minutes, and she ranted on and on about something they'd fought about the last time. Although he wasn't entirely sure of that, after all she was mostly calling him names and shoving diagrams and tables with completely obscure meaning in his face. Finally, she seemed to conclude from his display of complete confusion that he didn't know what she was talking about and therefore hadn't really meant any of the things he'd said. That made the whole speech kind of pointless and must have been why she settled on the next best thing to be angry about: How could he leave her just standing in the rain? Hadn't he noticed it was freezing and her clothes weren't really warm? If he wasn't listening to a word she said, couldn't he at least offer her his jacket or something? Why didn't he bring an extra one? After all, it wasn't really unusual that she came by all of a sudden! Didn't he at least care enough about her to do what was common courtesy?
What should he have done? It wasn't as if he could tell her. He'd never hear the end of "I thought you cared…". It would have been her favourite instrument to blackmail him into doing God only knows what. Yet, if he hadn't done anything, he'd probably be a dead man today.
So, at this point he'd ventured a desperate guess that this thing inside him might act like the Hollow. That it could make him stronger if he let it, up to a certain point yet to be explored, when it would have to be suppressed. As you might have thought, he knows better by now.
He kissed her then, quickly and probably without much grace, and his mind somehow short-circuited, face heating up beyond coherent thought, limbs moving without any semblance to coordination. It was a devastatingly helpless, wonderful feeling and he secretly wants to do it again. And soon. But he's found out that whatever this is, it's by far more powerful than the Hollow and if he doesn't want to hurt someone with it, he's better off not trying anything.
At least, Rukia had been quiet afterwards. She touched her lips in a wondrous way that made him feel even more funny, then proceeded to shove him with much less ardor than usual. At last, she smiled and he gave her his jacket.
They never talk about this, and he's glad. He really doesn't care to explain to her that, well, everything doesn't just "fall into place" when she's there. It's just that when she's not, something is off.