Because Naruto ate my brain. And because already I am seeing the fic world does not have enough strong Sakura. (The manga could rather use more, too.) And in the OT3, Sakura is key.

Three of Hearts

Correspondences:
The suit of Cups in the Tarot
Feminine
Sensitivity
Fulfillment
Healing
Harmony
Good fortune

The first time Naruto left, Sakura barely noticed. The second time, she could hardly breathe while he was gone – she had laid all her hopes on him, and he had grinned, and promised to return Sasuke. She had believed him. The third time he left, Sakura missed Naruto.

Missed him a great deal.

Naruto was the only one who still truly believed they would bring Sasuke back, believed it harder than even Sakura did. But she didn't need to see him to know that. In her darkest moments, when everything Tsunade-sensei was teaching her seemed beyond comprehension and she saw doubt and pity in the eye of everyone she met and the weight of her impossible (never impossible) task bore down on her, and she would have slumped into despair, always there Naruto's promise caught her, and she remembered that somewhere Naruto was working fit to kill himself day after day, and waking up stronger every morning. The memory pushed her back up, but it was not that inexorable faith of his that she missed (though it might have been nice, when the anxious glances of the other villagers seemed too much, to hear Naruto fearlessly shout their weighty looks into submission). His faith was always there. He left traces of it in everything he touched, herself included. And you couldn't miss what wasn't missing.

What she did miss was his obnoxious laugh. And she missed the way she had always been able to tell where he was in the village by the shouting and mischief that followed him around like the silent thundercloud that was perpetually floating over Sasuke's head. She had never noticed before how she had always kept track of Naruto and the position of his bubble of laughter and trouble. But she must have, to feel that absence so keenly now. It must have been an anchor, to leave her with this uneasy sense of being adrift.

Maybe before she had not noticed because Naruto and Sasuke had been gone at the same time, and always Sasuke had been at the forefront of her mind. She had known for years that she loved Sasuke. Her growing feelings for Naruto – whatever they were – had made her feel guilty, like she was betraying a little girl's dream of love. So she ignored them. But the feelings grew without her help – Naruto was like that. And somewhere along the way she began feeling guilty for not offering Naruto, whose own dream was nothing but acknowledgement from everyone he knew and cared about, more of her attention. And those guilty feelings got all tangled up with the old ones so she didn't know which was which and could never have sorted them out if she tried, which she hadn't, because she had still been pretending they didn't exist, but somehow it seemed that they were eased when Naruto and Sasuke were getting along, working together and exchanging smiles and even pretending they hadn't but knowing they had. When they fought, it was like someone had rearranged all the organs in her belly and then started poking to see what would be hurt.

She might have died, throwing herself between them that day, if Kakashi had arrived a few seconds later. But she couldn't think with them fighting – not sparring, not teasing, but fighting – like that.

And what was coming would be worse than that, Sakura knew. She and Naruto might well both have to fight Sasuke, and she knew that if it came to that their only hope was that they both be strong enough – neither one of them could bring him back alone. Sakura knew that Naruto would be ready, so she was determined that she should be ready too. Not just physically, but emotionally. Because the coming confrontation was not going to be easy, no matter what happened. She would be strong enough to face it, when it was time. She owed that to both of them.