Harry Sullivan wasn't quite as sane as one might think. It wasn't something that was really difficult to conceal, though. After all, anybody that met the Time Lord had to admire him. Besides, they all thought he was in love with Sarah Jane. They were partially correct.
He loved that fiery glint in her green eyes. He loved that dangerous smile that surfaced whenever there was trouble. He loved it when she linked arms with him as they walked for no apparent reason. He even loved the angry lectures she gave him when he called her "old girl".
The problem cropped up in the fact that Sarah wasn't the only one who he'd fallen for. Sometimes his memories and imagination took him elsewhere: A toothy grin and fly-away curls. That scarf, that hat, a handful of jelly babies. Pale blue eyes that were sometimes filled with laughter, but could be so very sharp sometimes. The Doctor had playfully called him an imbecile. How appropriate for the man with the hapless, stupid love for two people that didn't look twice at him.
He still wasn't sure why he had chosen to stay on Earth. No, he wasn't into running for his life every time he stepped out of the TARDIS, but he had borne it for Sarah and the Doctor. Sometimes they did need more than each other—Davros had unwittingly proven that. Both Harry and Sarah had been on the torture racks, the Doctor being threatened into giving up universe-changing information—universe-destroying information—and Sarah was too brave and too wonderful to ask for mercy. How could Harry allow the Doctor to watch as Sarah was tortured to death? How could he allow Sarah to be tortured to death? So he chose what he thought was right. Maybe it branded him as weak, but not everyone could be Sarah Jane Smith. And especially not Harry Sullivan.
Maybe he was deluding himself. They didn't need him—and didn't seem too sad to see him leave. The delusions were less painful than the truth.
Sometimes he can almost make himself believe that things were better off with him on Earth, working for UNIT.
And sometimes, he feels not only like the imbecile as he has been branded, but a coward as well. He let the two people he loved the most leave him behind.