Harry and Seamus have been sharing a dorm room since their first year at Hogwarts, which is why it surprises Harry so much to find out that Seamus seems to believe the Daily Prophet's claim that Harry is an attention-seeking, delusional fool who is lying about the return of Voldemort.
Apparently, Seamus's mother almost didn't allow Seamus to return to Hogwarts for his fifth year because she has been following the Prophet's account of Professor Dumbledore's faltering sanity. Seamus and Harry fight over whether or not Harry is telling the truth the first night they return to Hogwarts. Their relationship remains strained for most of Book 5. In fact, it isn't until Harry gives his interview with The Quibbler that Seamus approaches Harry to say, "I believe you. And I've sent a copy of that magazine to me mum" (26.111). Seamus's change of heart in Chapter 26 is one of the few wholly positive things that seem to happen to Harry in Book 5.