Harry should have shown more obvious signs of the abuse he endured:
Spending a decade of his developmental years in a tiny cupboard fucked up his senses. Harry gets overstimulated very easily—lights are too bright because he’s used to the dark, sounds are too loud because he’s used to things being muffled.
He gets claustrophobic because the cupboard was too small for him, but he also finds comfort in small, dark places because darkness meant safety. The cupboard was his prison, but it was also the only place small enough that the Dursleys couldn’t reach him. It was the only place he wouldn’t be forced to work, and he couldn’t be hurt while he was in the cupboard.
Harry has a habit of pocketing food at meals and stashing them in the bottom drawer of his dresser because he’s so used to having his meal privileges taken away at the drop of a hat. He’s gone his whole life hoarding food and being made to work for every crumb; he can’t bring himself to believe that food at Hogwarts is just given freely to anyone who wants it without conditions.
At first, he flinches when people raise their hands at him. When Hermione’s arm shoots up to answer a question, Harry jolts back. When Ron goes for a high five, Harry’s hands come up to shield himself.
He’s deeply mistrusting of adults and authority figures, because the only adults in his life have always hurt him. He’s especially terrified of Snape because he so clearly hates Harry, and seems like he would gladly cause him harm if he could.
For most of his life, Harry has been treated like a waste of space. He grew up hearing “we already spend far too much money on you” when the Dursleys were doing the bare minimum, sometimes not even that. So for the Weasleys, a family that can barely afford to put food on the table and books in their children’s school bags, to be so willing to take him in as another mouth to feed is incomprehensible.
It all comes to a head one day, and he snaps at Molly, completely overwhelmed by the amount of unconditional love he’s being shown. Harry can’t understand how they can be so kind to him and care about him so much when he’s wasting their money and taking up space they don’t have.