While it's not directly stated it's very heavily implied that his own sister has given him severe trauma. On top of this the other characters are shown mocking him for it most the time on accident, but this has still caused him to feel like he's being mocked or attacked even when he isn't like when Pinkeye tried to confront him about the effects Melvin's trauma was having on his life and instantly goes into defense mode. I would like to mention his trauma that most harmful to him mentally his sister's you were a conjoined twin story even if he knew the story was false it would still be creepy to think about but he genuinely thinks it's true plus he is pretty young this isn't a simple oh I don't like that either it's something that actually effects and nags at him we are shown multiple scenes of Melvin being triggered by the thought of the story he actually doesn't think he is safe in his own home.