Touya always knew that Yukito wasn't quite what he seemed, and after Yue manifests himself, he realizes that Yuki doesn't know about his alternate self (at least, not on a conscious level). The new school years finds Touya trying to tell Yuki something... whether it's the truth about himself or the fact that Touya is in love with him isn't something that's made too clear. Because CLAMP will be CLAMP. At this point, Touya doesn't fully understand all the implications of his best friend's peculiar situation, and he seems in no hurry to make Yuki realize the truth.
He probably feels they have all the time in the world to tell each other all their secrets.
This, however, is not the case.
Yue is a creature that shares the moon's nature. The moon can only reflect another's light, not generate its own. In a similar manner, Yue needs the power of his Master (or Mistress, in this case) to continue existing. This wasn't a problem when Clow was his Master, but Sakura is just a little girl. She simply isn't strong enough to sustain herself, the Cards /and/ the moon guardian. Little by little, Yue's light dimishes as he begins to disappear. And so does Yuki. He falls asleep constantly and tries in vain to satisfy an ever-increasing emptyness that manifests as hunger.
What's amazing to me is that with the very limited amount of information he had, Touya was able to put two and two together and realize what's happening to Yuki (Sakura, on the other hand, doesn't have a clue till the end). Time passes, and both the situation and Touya become desperate. Touya's voice is no longer calm and gentle when he tries to talk to Yuki. The thought of losing the one he loves is unbearable to him, he knows he has to do something to keep Yukito by his side, and at some undisclosed point he comes up with a desperate plan. But how can he help if Yuki doesn't even realize what's happening? He has never met Yue face to face, therefore he can't offer his help to the angel.
Throughout the series, Touya has been the very image of collected cool. A widening of Touya's eyes is equivalent to screams of surprise from anyone else. This all changes as he actually begins to see Yuki disappear before his eyes. It is then that Touya finally seems to open up, it's then that he goes beyond subtle displays of affection. Watching Touya craddle Yukito as if he could make him stay merely by holding onto him tightly enough, there can no longer be any doubt about his feelings.
Finally, there can be no more waiting. After a near-fatal accident (in the anime, at least), Touya refuses to be sidetracked any longer.
"I know you're not a human being, so you don't have to hide from me anymore..."
At his words, Yue comes to the surface. Touya tells him that if power is what Yukito needs, he'll give up his to save his friend. This is, for Touya, the ultimate sacrifice. It's not something he would have decided lightly. Giving up his power means never being able to see his mother again. It also means that he has to let go definetly of his sister. All his life he has watched over Sakura, and doing this will mean he'll no longer be able to do that.
For love of Yukito, Touya is willing to let go not just of his power, but of everything he has always held dear. This is the ultimate proof that for Touya, there is nothing and noone as important as Yukito.