Why does Seamus doubt Harry about Voldemort

Question

I can understand why people who aren't close to Harry disbelieve him when he claims that Voldemort killed Cedric, especially when the Ministry wants to spread the false narrative that Cedric died in a tragic accident, but why do some of his friends, like Seamus, honestly believe that Harry would orchestrate such a lie about Cedric's death and Voldemort? It's not as if Harry had a reputation to warrant the strong suspicion (on the contrary).

I can understand why fear and the propaganda is convincing people that Harry is lying, and in the case of Seamus, his mum's opinion is also a factor, but to have been friends (and in the case of Seamus shared a bedroom) for 5 years with Harry, who doesn't have a reputation of lying (in fact the opposite), and to still suspect strongly that he is orchestrating this dark lie about Cedric's death and Voldemort's return (for what, attention?) doesn't make sense to me.

“That’s out of order, Seamus,” said Ron, whose ears were starting to glow red, always a danger sign.
“Out of order, am I?” shouted Seamus, who in contrast with Ron was turning paler. “You believe all the rubbish he’s come out with about You-Know-Who, do you, you reckon he’s telling the truth?”
“Yeah, I do!” said Ron angrily.
“Then you’re mad too,” said Seamus in disgust.
_{—Order of the Phoenix, Chapter 11, The Sorting Hat's New Song}

Answer 1

Since we don't get a lot of insight into Seamus' mindset here, we have to do a certain amount of speculation. That being said, Seamus' reaction is marginally more rational than it might appear.

Harry doesn't make himself seem credible

It's worth noting that, initially, Seamus actually is interested in hearing Harry's point of view; Harry's hostile response is what turns him off (bold is my emphasis, italic from the text):

"Look...what did happen that night when...you know, when...with Cedric Diggory and all?"

Seamus sounded nervous and eager at the same time. Dean, who had been bending over his trunk, trying to retrieve a slipper, went oddly still and Harry knew he was listening hard.

"What are you asking me for?" Harry retorted. "Just read the Daily Prophet like your mother, why don't you? That'll tell you all you need to know."

"Don't you have a go at my mother," snapped Seamus.

"I'll have a go at anyone who calls me a liar," said Harry.

"Don't talk to me like that!"

^{Harry Potter and the Order of the Phoenix Chapter 11: "The Sorting Hat's New Song"}

Obviously we know that Harry is telling the truth, and that his frustration is at least somewhat justified, but Seamus doesn't. From his perspective, he's being open-minded: he gives Harry an opportunity to tell his side of the story, and is met with frankly shocking hostility. Regardless of what he personally believes at this point, the Daily Prophet seems like the more credible source; at least they're not making ad hominem attacks on his mother.

Which leads me to...

Seamus is quite angry in this scene

It's notable that, as we can see in the quote above, Seamus gets very angry once Harry starts making comments about his mother. As soon as this happens, his ability to have a meaningful, intelligent conversation is gone; as anyone who's spent any amount of time on the Internet will tell you, making someone angry is about the worst possible way to convince them of your point of view.

Even if Seamus had agreed with Harry before this - and it seems more likely, given his earlier comments, that he was undecided at best - he certainly wouldn't be agreeing with him afterwards. This is by no means a rational position, but it's how our monkey brains think; there have been studies on this in the context of political opinions, and (again) a cursory skim of the Internet will tell you that it holds for non-political opinions as well.

But all of this is in the context of a single conversation; what about more generally? Why does Seamus still have doubts when he gets up the next morning, presumably (slightly) less angry?

Harry still doesn't act like a credible source

Before the Quibbler interview, Harry does very little to make himself seem believable; his attempts to tell his story basically boil down to saying "nuh-uh" at higher and higher volumes, which makes it very easy to believe the story that he's lying, unhinged, or both. Take, for example, his exchange with Umbridge:

"So we're not supposed to be prepared for what's waiting out there?"

"There is nothing waiting out there, Mr. Potter."

"Oh yeah?" said Harry. His temper, which seemed to have been bubbling just beneath the surface all day, was reaching boiling point.

"Who do you imagine wants to attack children like yourselves?" inquired Professor Umbridge in a horribly honeyed voice.

"Hmm, let's think..." said Harry in a mock thoughtful voice, "maybe Lord Voldemort?"

[...]

"You have been told that a certain Dark wizard has returned from the dead -"

"He wasn't dead," said Harry angrily, "but yeah, he’s returned!"

[...]

"As I was saying, you have been informed that a certain Dark wizard is at large once again. This is a lie."

"It is NOT a lie!" said Harry. "I saw him, I fought him!"

^{Harry Potter and the Order of the Phoenix Chapter 12: "Professor Umbridge"}

And so on. Again, we know that Harry is right, but to the rest of the school he looks like he's throwing a temper tantrum; he acts deranged, so it's easy to believe that he is. This is a common marginalization tactic, and it's common because it demonstrably works.

And it's worth remarking that, as soon as Harry actually does tell his story, without raving like a lunatic about it, Seamus (and others) actually do start to believe him; Seamus apologizes on literally the same day the Quibbler article comes out.