Why do you think Sonic is so fast?

Sonic is only the speed of sound he still fast but not the fastest I'm a fan of Sonic myself but not a huge fan the fastest thing alive? HA there's few characters are faster then Sonic.

He says he can run at the speed of sound, yet, we haven't seen that. So I think that Sonic has run at the speed of sound ONCE, so he thinks he always runs at the speed of sound. But when I was reading some fan fiction, the guy wrighting it said that Sonic could be fast because he might of touched a chaos emerald at a yong age. Besides from that, I got nothin'.

I think he was born with it??

I don't think Sonic is fast, I know he's fast because that's how Sega made him.

Well we know that Mobius is Earth thousands of years in the future, and that Mobians (like Sonic) are mutants caused by the fallout of chemical and alien wars leading to an apocalyptic era prior to the mobians coming about.

As these are mutants it's very easy to suggest that they simply gained powers like the X-Men. I would think they would be a bit weak which is why we don't see very many until Sonic's time, but by the time Sonic is around Eggman's invaded, and pollution has started up again.

That combined with power rings and general Chaos Energy, would modify his already fast pace of speed, and increase it over time. Sonic's been exposed to Chaos Energy for years now, so his body has most-likely grown stronger and stronger from it, which explains why he's able to reach (or possibly surpass) light speeds if he pushes himself enough.