Why People Travel
I came across your question several years after you posted. As a travel psychologist who is currently working with other psychologists who are trying to answer this question, I would like to do two things. One is to note that there has been some research in the field of psychology that is supported by travel industry. The motivation is how to get people to travel using tourist services (e.g. airplane, trains, cruise ships etc.). There has been travel by engineering professionals who want to enhance the travel experience using different types of modes of transportation. Other social scientists (e.g. sociologists, psychologists, anthropologists) have examined the question from a variety of perspectives. In sum, the answers vary depending on the agenda of the research sponsors, the type, length and mode of travel and ultimate objective of the traveler. So there is no simple answer. The group with which I am associated, International Travel Psychology Consultants is building a web site now to address this question (along with commonly asked questions in the realm of travel psychology) for both consumers and psychologists. I will post the URL later this year when it has been published.
In the meantime you might want to look at some opinions published by Dr. Michael Brein. Just put Michael Brein in the search engine. While his responses are not empirically based he has been looking at this issue for a long period of time.