I don’t know how anyone can think this isn’t one of the saddest moments ever in new Doctor Who, at least.
The pain the Doctor feels doing this must be immense. And, to be quite honest, I think he closes his eyes so he doesn’t have to look at her and share her pain. He’s been forced to make her forget. With Rose, he had no choice at all, and neither did she. Martha chose to leave. To save the life of Donna, he does the thing he knows he’s going to regret. He never had a friend like Donna Noble, and he probably never will again. Another companion, another friend, lost.
Now look at Donna. That look in her eyes. Because, the thing is, she knows she’s going to forget everything. And although she won’t even remember forgetting, she’ll go back to being plain old Donna Noble. She wanted to stay with him, her best friend, forever. To travel the stars and see brilliant things. She loses everything in that moment, and she’ll never be the same again. Donna Noble will have that sad look in her eyes for the rest of her life. And she won’t even know why.