I don't just think, I personally believe in the existence of extraterrestrial beings or aliens as they are usually called.
The thought of aliens existing might seem far fetched during the middle ages, but with the overwhelming amount of knowledge we now have through modern science, I see no reason why we should doubt their existence. There's more than enough evidence around and above us to make the thought conceivable.
Milky Way which happens to be our galaxy, houses at least 100billion stars with trillions of planets in them. This is just as far as our technology can tell us for now, meaning the numbers might even be greater. Giving this fact, it's not un-logical or misleading to say or believe that Earth is the only planet with life form. The universe by the way has an endless number of galaxies with some 10x bigger than ours. It's way too large to think only humans are the intelligent life forms present in the entire universe.
If the argument goes further, some might point to the fact that only an Earth like planet can be inhabitable, making references to known planets like mars, Jupiter, etc, which are not life friendly because they lack atmospheric composition. What if it's just human psychology and we are probably wrong. The human brain and mind has been nurtured or programed to work with our natural phenomenons. Anything outside of that, makes no sense to us. That is the major reason why no one wants to subscribe to the idea of a different type of beings with the ability to survive outside our known natural habitat, the things that keeps us alive like oxygen for instance. It's possible that aliens in different planets have their own type of life sustaining atmospheres, which might not be favorable for humans if we were to live there.
By the way, NASA through it's Kepler space telescope launched in 2009 has discovered more than 2000 alien planets with a handful of them being Earth like. They are called the Kepler series. There's a high probability that these Kepler planets are life friendly and even possibly inhabited due to their size and rocky nature, which are some of the basic potentials for life friendly planets. NASA is already in the process of advancing the space telescope to a much higher degree in an attempt to have a better look at these planets, with a possible view of their inside. I can't wait to see what discoveries will be made.
Asides all said above, the internet is swamped with so many information suggesting the existence and presence of extraterrestrial intelligence around us. The most compelling ones are videos with some showing frequent alien visits to our planet. I've seen so many of them. Though some look like a made up film, it's hard to believe that they are all fake...especially the unedited clips.