Whatever your feelings on Donna as a character, I believe her exit from the series was the most tragic of all the companions we have seen so far. Yes, this even beats out Rose’s departure. When we first encounter Donna, she is more concerned with the latest gossip than with the well-being of others. She is annoying to most fans, and she is certainly a much different companion than Rose or Martha; she doesn’t seem to have much to offer to the storyline besides some sassy retorts. As her time in the TARDIS goes on, I see a change in her behaviors. She is still sassy, but some more compassion and depth develop in her character. Donna fights with the Doctor to save the people of Pompeii, something neither Rose nor Martha did to such an extent, and she understands the true plight of the Ood when the slavery plot is discovered. She understands that they are a trusting race, and the humans took that for weakness. When I saw this episode, I was shocked and impressed that Donna had such insights.
All of this makes her departure so tragic. Donna becomes a better person because of the Doctor and all he had shown her. To forget it all and go back to who she was before she met him is such a loss, both for her and for her family. For a brief moment, Donna understood that she was important and that she could do something to save the world. The Doctor does have a relationship with his companions, even if not truly romantic, and when they part, it is like breaking up. I think both the Doctor and his companion feel sad when they part company, but remembering the good times and how the experience changed them is an integral part of who they both are. If I had an opportunity like seeing the world, even if it couldn’t last forever, I would want to cherish the memories as part of who I am. Forgetting would be all the more tragic because I wouldn’t even know what I had missed.