The reason why Mathew talks in a whisper is because his voice is magic when spoken aloud. His voice can control any magical beings close by, any that hear him, and the problem with that is that nations certainly aren't human, nor is the company they keep like England and Norway's fairies.
The reason for his silence is simple. He doesn't want to hurt anyone. Not ever again. Not after what had happened last time. He had never meant to hurt his own brother. Not ever. All he could say now...was that he was lucky England was there and Norway had also been over for a visit, and that they had wiped everyone's memories of the incident.
He knew and they had known that he would have become a social pariah if anyone had remembered (once he momentarily gave them back their memories so that they knew the situation... so that they were more pliable and easier to control and commanded them to help him). So now he was the only one who knew about it, and yet he'd still managed to turn out to be a social wreck, just because he was always flinching away from conversations and others in general.
And it was all because of his darn throat. Well, his vocal cords, to be more specific.
Why couldn't he have had a more general magic control like the members of the Magic Club? Why did no one else seem to be afflicted by an unruly power?
It wasn't fair. It just wasn't fair...
His biggest problem with his magic to date had started earlier that evening. For once, the other nations finally saw him for a while, and they'd gone out to a bar, but though the evening had been going fine up until that point, Mathew soon found himself horrified before the evening was finished to find that he'd drunk too much and couldn't control his volume and spoke loudly, much too loud to keep his power under control, while at the bar. Big mistake.
To cut this explanation short, he had a bit of a-alright, maybe he'd had a very big crush on Gilbert for the past year or so, and he had talked loudly in his presence and his magic must have bled into his voice and made his inner wish come true, because Gilbert had started flirting with him when he never had, or ever even seemed inclined to do so before.
Mathew knew, he knew it so well that it physically hurt, that it was his magic that made Gilbert, who had only ever seen him as a friend, start flirting with him, and so he'd fled home. Even once home, he couldn't bear the thought of comfort, not after what he'd just done by forcing Gilbert to act like he had. As far as he was concerned, he had acted like a walking date rape drug, and he hated himself for that, so he'd yelled at his bear and made Kumajirou go crawling away when he'd come to investigate and saw him crying.
Canada felt bad, of course he did, for making his bear leave him, but he also knew he didn't deserve comfort. Why didn't he control himself better? He should have controlled his voice better. He knew better than to talk above a whisper. He knew better.
"I just don't want to hurt you guys," Mathew choked out as he stayed staring into the fire with tears rolling down his eyes.
And that was when Mathew decided that speaking quietly just wasn't enough. He couldn't have any interaction with his fellow nations. Ever again. It was just too dangerous, for them and especially for him. He didn't want to make anyone be forced to do anything. It was just immoral.
Mathew sniffled one more time, and resolved to call his Prime Minister in the morning to make the announcement that Canada would no longer attend any further meetings. He could just respond to emails and write notes after all...
With that decided, Mathew fell asleep.
And, the next morning, his Prime Minister was called up and the arrangements made up.
As far as Mathew was concerned, he would never see any of his fellow nations again. He would never see his brother again, his Papa, England...or Gilber-Prussia. He couldn't see any of the other Nations again.
Especially not Prussia.