You're right, we do tend to ride a bit faster than the limit. Sometimes there is a good reason for it, like on the freeway riding in grandma's kill zone, it's a place you don't want to be, so you speed up. The same applies when you're anywhere near someone texting behind the wheel, the safest place to be is about a mile in front of them.
Other times it's just speeding because the bike wants to go faster. It's hard to describe, you have this connected feeling to the machine you're on, the lightest twist on the throttle hurtles you forward, and you want to do it again and again. It's also that a bike is so much more nimble than a car, fast just feels right.
The entire experience is just plain fun. Bikers know why dogs like to hang their heads out the window.