What I like especially about Alistair compared to other cross-regen candidates for soulmates/true loves of the Doctor (Jack, Rose, Clara, the Master) ((although I love all of those too)) is that Alistair isn't cosmically important.
He is to his species on his planet in his country, as the leader of a British government organization. But that's not as important to the massive expanse of the universe. Plenty of other Alistair Gordon Lethbridge Stewarts on different planets have done the exact same thing as species come and go. In the end, he's a spec of dust compared to the infinite vastness of everything else. He's not particularly powerful or important to the universe at all. Just another mortal human man.
I'm sure if fate were slightly different, the Doctor would've let Alistair travel in the TARDIS for a year and then left. That's not what happened though, this man, one they sort of knew briefly, took them in during a traumatic exile. Gave them a job and a home and someone to treat as an equal when everything else felt so distant. Something their supposedly "superior" kind refused to do.
Also, I think Alistair is a good example of a companion where it feels like he's not subtly below the Doctor. The Doctor doesn't have a TARDIS to control where the humans go or any alien planets/alternate times to spout knowledge about. Alistair is a human, but also the Doctor's boss, but also their closest friend. They exchange ideas, they argue, they tease each other. And you forget you're watching a 700 year old vagabond criminal alien with psychic powers. Because around Alistair, it's not what the Doctor wants to be. Alistair isn't special because he's the Brigadier, it's because he's kind and intelligent, strong and loyal.
Whenever the Doctor comes back Alistair is always ready. In a time where their Brigadeir is dead, he haunts the Doctor's mind, every decision they make influenced by him- even when they don't even notice. The two of them are always together. In every incarnation of the Doctor, Alistair is there, literally and/or figuratively. They weren't destined by fate to be together but no stupid amount of fate can tear them apart.
The Doctor is lucky to love Alistair, Alistair is lucky to love the Doctor, the whole universe is a lot better off for all of it.
