Newt is gentle and always seem to know what to say to sooth Percival when he's in a mood or suffering from flashbacks, so Percival latches on to him and they develop a close/romantic relationship and start living together.

But when Percival recovers more, he either becomes too busy with work again or thinks he fell out of love with Newt because he can cope without him now, so Percival sends Newt away callously because he genuinely believes he doesn't love him/doesn't need him in his life anymore.

It's fine the first few days, but then Percival somehow feels off empty cold lonely when he arrives home and doesn't see Newt, but chalks it up as habit and it'll go away once he gets used to living alone again. Gets used to not seeing Newt's smile not being in Newt's presence.

Because it's not like he loves him, right? Wrong.

It doesn't get better. It gets worse.

+ Percival tries to reconcile but Newt lets Percival know exactly how heart broken he is.
++ Queenie giving Percival the "You're a taker, Newt needs a giver" speech.

Happy ending optional :D