stephen's law:

when someone experiences something for the first time, they assume this is the first time this event has ever occurred.

Examples:

I get on a roller coaster. It's a thrill because it's a new experience and I subconciously consider the possibility that I am somehow in danger. To the guy running the roller coaster, it's not so thrilling because he's been working at the same damn amusement park for six years.

I'll post more examples when I think of them.