I just finished Planet of the Spiders and. im. not. okay. How did we get here. 3 went to Metebelis III and got this beautiful-looking blue crystal. He gave it to Jo as a wedding present. He gave it to her, this wonderful girl he didn't like at first but got attached to. They departed in silence, knowing they wouldn't see each other again (for a long time).
And now, this crystal that he gave her turns out to be dangerous. Taking that crystal in the first place turned out to be disastrous. So then he goes to hand it over to the Great One which destroys everything and everything is alright again, except it isn't, cause he's dying. He's dying because of his own mistake.
And it's almost a callback to what the Time Lords did to him. In his second body he was forced to regenerate against his will. His body was tampered with against his will. And it happens again with the Great One, who makes him move against his will, and says, "Is that fear I can feel in your mind? You are not accustomed to feeling frightened, are you, Doctor?" He's being used again, and it frightens him enormously, to the point there's a tear in his eye. It's not only hurting his pride to be used like this, but it's an exact reminder that it has happened before.
But then, as he dies, where does the TARDIS bring him? Where does the TARDIS bring her thief? She brings him home. "The TARDIS brought me home."
He was exiled to Earth alongside his forced regeneration, and he was very much against it. He hated being stuck in one place, he was a Time Lord with a TARDIS that could go anywhere in time and space and his freedom is taken away from him. But he starts to enjoy it more. ("That's where you met me." "That's right. That's where I met you. And that alone made the exile worth while, Miss Grant.")
Earth became more than the planet he was stuck on, the planet he constantly saved. Earth meant Jo. Earth meant the Brigadier and even Sergeant Benton. Jo saved him. Sarah Jane saved him. Earth became home. He saved his home, at the cost of regenerating.