Then I remember the episode Reunion. Where Rose freaked out because the Doctor had never even mentioned Sarah Jane Smith to her. Never even said her name before. That was her biggest fear: that the Doctor would someday not think about her. And then, she meets Martha and the first thing she hears is “Oh my god. He found you.”
Imagine that. Imagine if the first time you met someone, they immediately say “Oh my god. It’s you. Finally.”
In that moment, she knows how broken the Doctor was when they got separated. How even his future companions knew how utterly destroyed he was. Because he does not wear his emotions on his sleeve. He buries his pain. And for Martha and Donna to know how hurt he was. Well that’s a big deal.