they each had major PTSD from the Doctor.
Personal growth making or however one justifies it, nevertheless there’s a REALLY limited pool of people that will “get it” on earth after the doctor is out of their lives.
Mickey the pining after Rose whiner and smeared by her mother Mickey would not work with Martha.
but Martha after she traveled the world for a year spreading rumors about the Doctor, looking like a post apocalyptic and Mickey the rag doll tossed around even to a parallel universe but who saw war and taught first hand and clearly emotionally scarred in the process of “toughening up” himself but managing — well, the only flaw was them not explaining every bit for us.
In particular look at what happened to Martha’s family.
i didn’t expect it their pairing but it wasn’t shocking. tiny pool of emotionally supportive candidates for each other.