Because the thing is, Harry liked Tom. A lot. He’d entered this marriage agreement for purely practical reasons, wanting to make sure Voldemort never existed, and while he still cared about that outcome, now he also wanted Tom, just for himself. And he thought Tom felt the same. Tom had explicitly said he liked Harry and wanted him romantically. And Tom was possessive over him, unnecessarily so, and spent all kinds of time hanging out with Harry when he didn’t have to. Hell, he’d agreed not to kill the Riddle’s just because Harry had asked him not to! That meant something – right?