Ghosts
I wonder if Minato reminded Tsunade of Dan in the same way Naruto reminded her of Nawaki. Not romantically, just the fact that Minato has Dan's face, gentle demeanor and the same dream of becoming Hokage. Even if they were not close, I believe his death must have hurt her. In fact, both Minato and Naruto are a mix of the most important people in Tsunade's life: as I said, Minato has Dan's looks and personality, plus Jiraiya's hair and his view of the shinobi world. Naruto is also really similar to Nawaki and shares a lot of traits with Jiraiya. The same can be said about Mito and Kushina if we want to expand on it: both Uzumaki, married to a Hokage, jinchurikis of the Kyubi and with a strong personality. I know this is all due to Kishimoto copy pasting his own characters with no regards about creating new ones (this is particularly infamous with Naruto and his million copies), but uggg, there's something about this that's really interesting.
The Namikaze-Uzumaki family must feel like ghosts of the past to Tsunade.
