Theseus is a hugely overprotective big bro. Clearly most people cannot be trusted with his precious baby brother who must be kept safe. But his bestie Percy is an auror, with upstanding ethics, and a loyal personality. Obviously, they're perfect for each other.

Newt and Percival have spent so many years putting up with Theseus' increasingly insane attempts to get them together that when Tina brings Newt to meet "Graves" about his animals and the man doesn't immediately complain that he doesn't have time to deal with more of Theseus' mathcmaking that something is very very wrong.