The Doctor had never met anyone quite like Captain Jack Harkness, perhaps because there wasn’t anyone else like him in the entire universe. If there were… it didn’t bear thinking about. Nobody would be safe!
The man was an incorrigible flirt, completely indiscriminate. He considered every intelligent being he met to be fair game, and if his flirting was favourably received, so much the better. If not, he either tried harder or went looking elsewhere, meaning the Doctor either had to go in search of his wayward companion or leave him behind.
Better to keep a close eye on him.