Rose knew as soon as the TARDIS had disappeared she'd made the wrong decision. The sinking feeling in her heart was a dead giveaway. And Mickey... Mickey had turned out to be - well, not half the person she thought he was. No help at all, panicking, telling her to leave the Doctor and save herself. Calling the Doctor a thing, after all he'd done.
And what was she? She'd saved an alien's life, saved the world. What did that mean now? She'd thrown it all away out of a sense of obligation, of duty, to Mickey and her mum. But she'd wanted to go with him. Wanted to leave all this behind, travel with him, run in and out of danger with him, her hand in his. Wanted to live.
Then she heard the sound again. She'd come to know what it meant, now. Her heart leapt, and she turned around, seeing the TARDIS appear again, a wild, fierce joy surging through her. She barely even heard what the Doctor was saying, knowing what her answer would be, had to be, could only be. Saying a hurried goodbye to Mickey, she turned and ran, seeing in the Doctor's eyes that same freedom, wildness, joy that was in her heart, ran to him, joining her hand in his again, where it belonged. This is where I belong, she thought as the TARDIS door shut behind her. With him.
She grinned, and squeezed his hand, heart lifting as he turned to the console, picked something up, and turned back to her.
"Right then, Rose Tyler - you tell me, where do you want to go? Backwards or forwards in time. What's it going to be?"
Yeah, this was where she belonged. With him.