Humankind's fascination with space travel is a continuation of our collective desire to explore the unknown. Humans are caught between two conflicting desires: the need to settle and call a place "home"; and the desire to go "where no one has gone before". However, necessity has historically driven cultures to move beyond their familiar surrounding to find and explore new frontiers.
The colonization and settling of North America are a prime example of the desire to explore and expand beyond the known. While the actual discovery of North America may have been an accident, the original explorers and traders were looking for new places to buy and sell goods as their local markets were maturing. This initial stage of commerce paves the way for colonization and settlement by giving people a reason to come to a new land. Once the footholds were established in North America, the European peoples pushed forth, along with new Americans and other groups, to explore, conquer, and settle the new lands. The same has occurred in virtually every other inhabited continent in the world.
Today we possess a staggering amount of information about the Earth. The Earth's surface, the place where we live, does not hold many surprises for us. By and large the best and most bountiful lands have been identified and claimed. Meanwhile, population explosion and overcrowding is epidemic in some parts of the world. Over time the same thing will occur around the world. We will be forced to acquire more territory just to continue growing our species.
Space travel offers a way to return to the "frontier" era. Granted, it is currently a staggeringly expensive endeavor, but travel in outer space, including the planets of our solar system and other star systems within our galaxy, and the immensely large number of galaxies in the universe, represents a mysterious yet tempting new territory to explore. Popular culture has largely glamorized the possibilities of space travel through stories of beautiful people using amazing technology to explore this new frontier. Even the current space programs have an allure despite their relatively primitive nature.
In short, we have romanticized both the exploration of space and the experience of space travel through our myths and stories. However, this "branding" of space travel as both exciting and fun is in line with previous exploration movements in our history. You might say that our current fascination with space travel is merely a return to our roots, where we again have to struggle with the urge to explore and the urge to settle. Space travel looks like the next best way to head into the future by revisiting our ancestoral urges.