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He never meant to fall in love.
She was so unbearable. He hadn't wanted an assistant, confident that he could have managed just fine on his own. She had just been thrust upon him and she was quite possibly the most annoying person he had ever had the misfortune to met, always showing him up, always patronizing him. She acted like she was so much better than him and it drove him crazy.
She had such a sense of wonder. The look in her eyes every time they landed on a new planet, that look of awe and wonder. He recognized that look, it was the same way he felt when he had first ventured out into the universe. He had to admit having someone as knowledge as her around was immensely useful. Maybe this would work out after all.
She never meant to fall in love.
He was so irresponsible. They had a job to do and all he cared about was playing chess or fishing or some other nonsense. He barely scraped past at the Academy and yet he acted like he knew everything. He was the most arrogant person she'd ever met. Why the White Guardian sent him she just doesn't know.
He was so unpredictable. On Gallifrey, life had been pretty much the same, day in, day out. But with him everyday was an adventure, she never knew where they would end up next. He showed her a new way of living, showed her just how big the universe really was. He made her want to laugh. She'd never felt like this before.
For them, falling in love was perhaps the most beautiful accident.