You may be wondering, "why Touya and Yukito? Why not Duo and Heero, Trowa and Quatre, Kurama and Hiei, Subaru and Kamui, or any host of other yaoi couples?" Well, the answer to that question is fairly simple: because I like them best. ^_^v But if you're looking for a more in-depth answer...
Let's start at the beginning. ^^ I first saw Cardcaptor Sakura in the fall of 1998. It was recommended by a friend of mine, and at first I was sceptical. We had just done a monstrously large trade (but I got all of St. Tail! Wai! ^_^v), and I was sort of reluctant to get anything else. Plus I had no idea what it was about, so I'd be going into it blindly. But he assured me it was right up my alley (I am, after all, a huge fan of shoujo anime, and mahou shoujo anime in particular). So I agreed to the first four episodes...and I've never looked back since (now I have everything save the 2nd movie). ^_^
Anyhoo, to get back to Touya and Yukito... I got the first 20 episodes rather quickly (these were mostly all raw tapped off Japanese TV), and tankoubon 2-6 of the manga in Novemver 1998. I had got an inkling watching the anime, but the manga confirmed it: Touya and Yukito were a couple. Wai wai!
My initial attraction to them...well, I think it was because they looked so sweet together. The pet names, the looks, the touches...they all added up to major excitement for me. ^^ The characters themselves are great, too. Touya's rough, but gentle, and Yukito's sweet, but serious.
When I got the manga I'd always be chomping at the bit for more (Nakayoshi should be weekly!). Every issue seemed to have more and more Touya and Yukito content, sending us fangirls into fits of excitement ("OH MY GOD!! They HELD HANDS!!" :P). Their relationship seemed to be building up in a crescendo -- in the Sakura Card series, when Yukito started disappearing, Touya would frequently say, "Ore wa omae ga..." only to be cut off by Nakuru (the evil one >_>). Of course, us being fangirls, we assumed he was trying to say "Ore wa omae ga suki da" (I love you), but he was actually trying to say "I know you're not human" (Touya had known about Yue all along). I think we all collectively went "gahh!!" ;) But it was okay, because mushy things still happened. ^_^v
This is actually when Touya showed he loved Yukito without a shadow of a doubt. Touya gave his powers to Yukito so he wouldn't disappear. He sacrificed himself so that the one he loved could live. It was the greatest gift he could ever give, yet it was given so freely, without any need to be compensated, like it was the most natural thing in the world. Yukito, of course, felt incredibly guilty afterwards, but Touya would have none of it, constantly telling him that being able to see him was the most important thing.
I think the best moment was definitely when Sakura admitted her feelings to Yukito. Their conversation, after Yukito gently let her down, went like this:
Sakura: Yukito-san, you also have a person you love best, so that's how you knew, right?
Yukito: That's right.
Sakura: Is it oniichan?
Yukito: ::he was *very* shocked:: ...yes.
And we all collectively cheered! ^_^ That was *very* exciting.
They never kiss or do anything remotely sexual (damn...); their relationship depends on looks, touches, and their devotion to one another. The rest is up to the imagination of the fangirl. ^_^ Yum...chocolate sauce... ^o^
So I guess my reasoning can be summed up as follows: their personalities blend perfectly, they show they love each other in the cutest and truest ways, and CLAMP shows us just enough to have us begging for more. *_* They're perfect.