The question of whether we are alone in the universe is actually relatively new to mankind. Before Copernicus and his geocentric theory of the universe, man was viewed as the center of divine creation and the earth was the center of the universe. From Aristotle down to Copernicus no one knew of other planets or a massive universe as everything believed to be above us was crystalline.
The church for more than 1500 years taught the same. God created the earth as the center of the universe and mankind was the center of that creation. Any other thinking was met with swift retribution, and many times with death. Copernicus did not publish De Revolutionibus until he was on his death bed for fear of being burned at the state.
In the 1600's Galileo would, for the first time, turn the telescope to the heavens, and see for the first time, the existence of planets, stars, and other heavenly bodies. He spent the rest of his life under house arrest by the church for teaching Copernicus' theories. Galileo's ideas may have opened man's thinking to the possibilities of other life forms. If there were planets out there, what prevented the existence of other living things?
In the 1900s, radio programs raised the specter of an extra-terrestrial invasion from outer space. The programs were so life-like as to cause widespread panic. Even today the fear of the unknown continues to cause speculation about the possibilities of alien life.
In the twenty-first century we benefit from further space exploration, and have a better grasp of our immediate universe. Probes to our moon and other planets like Mars, once again raise the ideas of life on other planets. We surmise that for life to exit water must be present. Remnants of ice on Mars cause speculation of the possibilities, but they discount the fact that another form of life might exist that does not need water.
To date, no definitive evidence proves that aliens have been to our planet. Unfortunately, those who say otherwise always seem to be on the fanatical fringe of science or society. This tends to discredit any real findings that might indicate otherwise.
Several factors must be true for other life forms to exist, and be able to travel. They certainly would need to be more advanced than we are to be able to travel through space. One of the great handicaps we have is that of space travel. It has taken more than a century to move from airplane flight to space flight. The space station, while a great step forward, is but a stepping stone to moving into actual space travel and is slow and tedious. Time, food, oxygen, and all the other issues to sustain life over a long period of time are problems that must be overcome.
Whatever your thoughts on this subject, the presence or absence of any credible evidence to prove their existence makes for a grand time of speculation, and we will enjoy the possibilities for years to come. We will get serious if, and when, aliens land one day on our doorstep.