Collide.
He proberly should have known, to be fair. He was a Time Lord (Gallifrey or no Gallifrey) and as such, one tiny little human shouldn't have had what little Time Sense he had left spinning until he felt dizzy.
But.
Running across London Bridge with the houses of Parliament behind them, the Doctor found it hard to care as he realised that he had no ideas as to what the girl's- Rose's- future awaited.
!*!*!
A few months and a lifetime later, a new-man-who-was-the-same-man finally felt the pressure of her lips and her hips and her chest against his, and he should have stepped back, warned her off, and saved them both.
But.
She hit like a hurricane, and even though she'd leave just as much destruction and grieving behind as a real one, it was hard to care when she kissed like her life depended on it.
!*!*!
Several years later, he finally got the girl for keeps, for the forever she had promised and he had so desperately wanted. He should have been happy, ecstatic even, knowing that she could spend the rest of her life with him.
But.
It was hard to be happy when the love of your life was kissing you but it wasn't really you kissing her back, leaving you to leave her for good.
!*!*!
Five grey hairs and two wrinkles later, not only was he sure that he had received Donna's aging genes, but also her loudness and distinct lack of fondness for queues. He couldn't figure out why Rose would stay with him.
But.
She had promised him that she really would stay forever and ever, as long as he promised to never try and call her "Snookiebums" (in public or not) ever again.