From NPR, via Andrew Sullivan:

Have you noticed, [Neuroscientist David Eagleman] says, that when you recall your first kisses, early birthdays, your earliest summer vacations, they seem to be in slow motion? "I know when I look back on a childhood summer, it seems to have lasted forever," he says. That's because when it's the "first", there are so many things to remember. The list of encoded memories is so dense, reading them back gives you a feeling that they must have taken forever. But that's an illusion. "It's a construction of the brain," says Eagleman. "The more memory you have of something, you think, 'Wow, that really took a long time!

I think there is a parallel between this and peoples' ruminations on living abroad. Stimulation distorts time and space. I can think of weekend trips that I had to truly fascinating places, and in the short time I spent there, I had so many experiences etched into my memory that in retrospect I marvel at the shortness of my stay. It's also similar experience to when you quickly fall for someone romantically, and you can't believe that you've only known each other for a few days.