We would have to be pretty naive to think that we are the only intelligent life force out there. However, I do not believe that aliens are these bug-eyed beings who cruise around in a ship. You have to first understand that humans define living beings as beings that breathe in oxygen, change and grow, reproduce, etc. But what if there is another form of living that we cannot comprehend? So, we may think that many planets are inhabitable because they have no oxygen and whatnot, but if you think about it, people are most likely referring to the planets in our solar system. There are actually millions or billions of planets in our universe. There might be some form of life on one or more of those planets. Now, we are also very egocentric so when people think about aliens, they visualize them with human qualities. Perhaps there are other life forms out there that are incomprehensible to our very human minds.