The Doctor, in various incarnations, has been around for centuries, travelling from world to world and throughout time, fixing things that have gone wrong. Gallifreyans are naturally longer-lived than many of the universe’s sentient races, and as a Time Lord he also has the ability to cheat death by regenerating.
But the thing about living so long is that the regrets pile up. Things he’s done that he wishes he hadn’t. Things he didn’t do that he now realises he should have. People he’s let down or failed…
Worst of all are his companions; so many people he left behind.