Because Thor just had to have gone to see Jane again when he came back to Earth.

It was late. Probably really late, but Jane couldn't bring herself to care. She just had to finish analyzing these last groups of readings, and then she could go to bed. "Just one more," she whispered to herself, reaching for the printed pages.

"That's what you said two hours and four readings ago!" Darcy complained from her spot on the couch.

"And I said you could go whenever you wanted," Jane countered, searching her desk for a pen. "Dang it, I just had one!"

"It's right here," Darcy said as she came up behind Jane and plucked a pen from its spot in Jane's messy up-do. "I'm going to bed, and so should you."

"Yeah, in a bit," Jane said absentmindedly as she took the pen from Darcy. Her attention immediately went back to the papers in front of her, and she was so focused on them that she didn't even notice the other girl's departure. But after another hour had passed, she finally had to admit defeat. There was nothing in the readings that pointed to this recent storm as being anything but ordinary. It was another dead end.

"That's ok, maybe tomorrow," Jane told herself. She couldn't give up, she just couldn't. He said he would return, so she would keep searching until he did.

"Right, time to re-set," she told herself, getting to her feet with a groan. She had to go to the roof to re-set the instruments…but the couch just looked so inviting. "Nope! Sleep after," she ordered.

She didn't know how, but she made it upstairs and re-set everything in preparation for the next storm. Then she took a moment to lie back in a lounge chair and just gaze at the sky. Just five minutes, she thought. But it was oddly comfortable, and the weather was so nice, that before she knew it, she was drifting off to sleep.

If she had only waited a few more minutes, sleep would have been the furthest thing from her mind. As it was, she was completely oblivious to the presence of her late-night visitor.

Thor touched down gently on the roof, going without his normal stormy entrance. He hadn't wanted to draw too much attention, and now he was glad he hadn't. He knew how much Jane needed her sleep these days.

"You really work yourself too hard, my sweet Jane," he whispered as he sat down next to her. He didn't want to wake her, but he couldn't resist reaching out and grasping her hand. Her fingers curled around his reflexively, and he smiled.

"I know not if I shall see you again during my time here," he told her, his voice the barest of whispers. "And I wish I could say that I knew how future events will transpire, but I do not. But I do know that you will see me again. I promise you that, Jane Foster. I will find my way back to you."

He leaned forward and brushed a gentle kiss on her forehead, staying there for a few seconds to fix the moment in his memory. Then he scooped Jane up in his arms and headed back downstairs.

He moved as smoothly as he could, but he had to freeze once on the steps when she stirred in her sleep. Luckily, or not luckily, she didn't wake up. Finally placing her on her bed, he pulled the blankets over her. He chanced another touch, reaching out and running his fingers down her face as a farewell. She shifted once more, letting out a murmured, "Thor," before settling back down.

And then Thor left, ready to go and face his Brother and bring him back to Asgard before anyone got hurt.

And when Jane woke the next morning, she figured she must have walked herself back to her room in a half-sleep daze. But throughout the day, without really being aware of it, she kept brushing her fingers over the spot where Thor had kissed her.