In my opinion, this is the reason Journey's End ended like it did.
The first day, they went to Barcelona- the planet Barcelona- and ended up stopping a mad genius from genetically engineering super-warrior dogs with no noses. Well, actually Donna did most of the stopping, joyfully yelling technobabble as she went. By the end the mad genius was pretty much just standing there with his mouth hanging open as she yelled at him. The Doctor watched with a slight smile on his face.
The first night, Donna dreamed of the death of planets. Of the fall of Arcadia. Of war.
"How do you bear it?" she asked the Doctor when he came in and sat at the foot of her bed, unusually quiet for once.
He looked up at her and grinned. It wasn't a very reassuring grin, she thought, seeing herself reflected in the dark pits of his eyes. "I don't sleep."
The second day they visited the planet of the butterfly people and didn't correct the butterflies when the insects assumed they were a couple. "The DoctorDonna, that's us," they chorused cheerfully in tandem.
Donna cooked a Tercerian meal for dinner that night.
The next morning the Doctor looked up from where he was tinkering with the console to find her staring at him with accusing eyes.
"You abandoned her," Donna said.