The Reason for Tears is 13th chapter of Tsubasa: Reservoir Chronicles.
Practically brimming over with excitement, Syaoran runs back to the Arisugawa household, eager to bring Sakura back one of her feathers. He barely acknowledges Sorata and Arashi, who notice his happy smile and realize that he's found it! He quickly rushes to the sleeping princess, and gently lays the shimmering feather on her, praying that she would wake up. Syaoran clasps her hand, remembering her sweet smiling face -- when she asked him to hold her hand so that he'd be the first person she sees upon waking. The feather is absorbed, and Sakura's eyes flutter open.
Syaoran smiles at her with a combination of ecstasy and relief, "Sakura!" However, her first words broke his heart: "Who are you?"
As the witch of dimensions had said, the price of retrieving the feathers was Sakura's memories of Syaoran. He would remember their shared past, but she would have no recollection of him nor of their past. Syaoran releases her hand as all the events of the day come tumbling down on him. He didn't even hear Fai or Kurogane arrive back at the house.
Gritting his teeth, Syaoran puts on a brave face. He introduces himself and tells her that she is Sakura, a princess. As she's lost her memory, she's travelling with them between worlds to retrieve what she's lost -- and that he's just one of her travelling companions. Sakura wonders aloud why he would do all that for her, a stranger. Fai, feeling the emotional weight of the situation becoming too much for Syaoran to bear, interrupts by introducing himself, Kurogane, and Mokona. While they get to know each other, Syaoran quietly slips out of the room, not unnoticed by his friends.
Sorata had thought that the weather looked ominous, and as expected, it started raining. Syaoran didn't seem to notice, as the raindrops hid his tears. Fai and Kurogane know that it must've been difficult for him to put on a smile, fake though as it was, when his most important person couldn't even remember who he was. Strong as Syaoran is right now, they know that if he wants to keep from crying, he needs to become even stronger. But then again, it already takes a certain strength to be able to cry when one should.
Outside, the three kudan shelter Syaoran from the pouring rain.
In another dimension, a girl mentions that they've found yet another feather. The man says that they may have been lucky now, but the next time may not be as easy. All he wants is the power to span dimensions, and he will get it, no matter how much blood needs to be spilled.
In a glass tube, a mysterious shadowed figure floats ominously, almost waiting and biding its time.