I think it's the way our brains deal with novelty. I also think this is why a period of time seems so much longer when you're little than when you're grown up.
To the extent my subjective time perception skews during a journey, it skews towards perceiving the return as taking longer than the departure.
The funny extreme comes from my Alaska vacation of a year or two ago. The trip to AK involved a long flight to Calgary and about 2 weeks of driving through Canada and AK. The trip back was by air from Anchorage and even with the delays, busted connections, and so on it only took about 30 hours from entering the Anchorage airport to unlocking my front door.... but subjectively the return seemed to take far longer than the trip out.
Usually my perceptions are just about dead on though. I know my drive to work in the morning is going to take about half an hour, and my drive home in the evening will take about an hour, and my perception matches that reality.