There's More to Doctor Who Than Meets the Eye!!! (Theory Time)
Doctor Who secretly follows the same pattern every season.
Each Doctor starts their season missing a specific human value, which is usually because of trauma or something they lost before that season even begins and the entire season becomes about rebuilding that value through companions, friends or lovers (wtv you wanna call it) without the Doctor even realizing it.
Season 1 with the 9th Doctor is an example for this.
After the Time War, the Doctor has completely lost the ability to truly bond with people. He helps people and then leaves like nothing had happened. He takes companions with him sometimes but never really connects with them long-term.
Untill Rose Tyler shows up.
At first she’s just useful to him but over time she starts changing the Doctor. He starts worrying about her, protects her differently, hesitates when she’s in danger, actually forms a bond with someone again.
That’s why Season 1 feels less like a story about aliens and more like a story about the Doctor relearning connection.
The same thing started showing up everywhere else.
Different incarnations seem focused on rebuilding different damaged values such as Connection, restraint, self-forgiveness, hope, responsibility etc. and the neat part is that Doctor's companions don’t intentionally teach him these things, they just live around him and slowly change him overtime.
Once the Doctor fully regains that missing value, that incarnation can’t continue anymore.
That’s when regeneration usually happens.
He keeps the memories but loses the body and identity that learned that lesson.
Basically that, perfection becomes too heavy to carry. The Doctor can’t fully embody every value forever without eventually breaking:
He can help people but he can’t become perfect or “God-like”
Tardis also fits this theory so well as the image of the God himself in the universe of Doctor Who.
It never really takes him where he wants to go, it takes him where something is broken, damaged, or about to fall apart, as if the Doctor’s role in the universe is just to repair things while people unknowingly repair him back.
It starts a story about constantly rebuilding lost parts of yourself over and over again.