I've always believed that time goes by fast because you follow a routine, and because there isn't much variance in your life.
Take me for example - I've been at the same company for five years now. Been in the same city for five years, known the same people. It seemed to have passed by way too fast.
On the other hand, a friend of mine has been working for 6 years, but he has worked at four different companies in different industries (IT, management consulting, tech). He has lived in New Jersey, NYC, London, San Francisco, and now Seattle. His six years must have felt incredibly long.
Why? Because of variance. Because every day is new. Because there is no standard long term routine.
Problem with most people is that they get into a strict routine. Everything goes into autopilot. There are few to no new experiences.