Donna watched.
She was always watching, waiting, trying to pick and pry at the man they called 'The Doctor'. Since their journey began, she had been trying to determine what made him tick, why he thought the things he did, what made him so reluctant to talk about far-gone events of the past, and why he was so afraid of the Daleks. But that day in Norway, on that cold, desolate beach, Donna understood.
Often, throughout their adventures, he would mention another. 'Rose,' he would always say, 'I remember this one time…' and Donna would sit, and and listen, and deep down be in envy of all the wonderful and amazing things that this Rose woman was able to accomplish. She often wished she had been brave and ambitious like Rose, unafraid of the unknown, ready to jump head-first into the fray. Instead, she would wait. And watch. And follow the Doctor instead of trying to take the lead. She had, in all due credit, managed to befriend the Doctor in a way that Rose had not: purely as a confidant. He had confided many a tale, many a secret and hard-fought emotion to her, and she, in return, attempted to explain away all of her insecurities and shortcomings in her mediocre life.
But here, standing on that cold beach, shivering against the tide winds, Donna understood why Rose was so important to the Doctor. Why they had fought so hard to find each other again, and why his face had lit up like her grandfather's Christmas tree when he saw her standing atop that hill in those war-torn suburbs. Donna watched now as Rose begged and pleaded, watched the Doctor fight himself, whether tearing apart an alternative universe was worth it if it meant having Rose by his side once again. Donna watched that first, heart-breaking tear roll down Rose's cheeks, watching as the Doctor's gaze follow its silver path. She watched his mouth form a thin line of regret, shut from saying everything that she knew needed to be said; watched as he tried to explain away his own shortcoming, why he was leaving her behind with him.
In the end, Donna watched Rose embrace the wrong Doctor, the one she knew would replace her friend in the girl's heart, and make Rose happy until they grew old together and passed. Donna watched her dearest friend tear his own heart to pieces, in the hopes that it might repair those hearts so savagely broken by the things he had done.
In the end, Donna watched the Doctor's eyes crumble underneath her own pathetic pleas as cold, calloused fingers brushed against her temples, and one last fleeting apology floated through the chilly air.