Doctor Who: 10 Defining Tenth Doctor Moments
7. Some People Live More In Twenty Years (The Lazarus Experiment)
"Some people live more in twenty years than others do in eighty. It's not the time that matters, it's the person."
Each Doctor has moment where they offer some morsel of wisdom that gets remembered long after they pass on the TARDIS key. For example, Three has "Courage isn't just a matter of not being frightened, you know. It's being afraid and doing what you have to do anyway." which frequently appears on memes featuring his Doctor. There's a good chance that this line from 2007's The Lazarus Experiment will be one of Ten's. It engages with the idea of living life to the fullest and, in a manner completely fitting of the Doctor, reminds Lazarus and viewers that having time doesn't matter, it's what you do with it.
The confrontation with Lazarus does make the moment even better. Viewers got to see a battle of wits between Tennant's Tenth Doctor and Mark Gatiss' Professor Lazarus as the two try to reason with each other, the Doctor trying to prevent what Lazarus is doing in reversing the ageing process and Lazarus arguing for doing so. It's the best part of the episode and sticks in the mind, being memorable even after the end credits roll.