I'll tell you one reason why we might be the only intelligent life, it has to do with the uniqueness of our environment. First off, our galaxy.
The Milky Way Galaxy is a spiral galaxy, our solar system resides on the orion arm of the galaxy, and our sun orbits around the center of the galaxy, which is significantly less elliptical ("eccentric") than those of other stars similar in age. This circularity in Sol's orbit prevents it from plunging into the inner Galaxy where life-threatening supernovae are more common. Moreover, the small inclination to the galactic plane avoids abrupt crossings of the plane that would stir up Sol's Oort Cloud and bombard the Earth with life-threatening comets. In fact, the Sun is orbiting very close to the "co-rotation radius" of the galaxy, where the angular speed of the galaxy's spiral arms matches that of the stars within. As a result, Sol avoids crossing the spiral arms very often, which would expose Earth to supernovae that are more common there.
Moreover, our solar system itself is special. We seem to find most star systems contain a binary star system, where 2 stars orbit each other/a center mass. That makes it increadibly difficult for planets to exist where life is possible due to extreme temperatures. Add to that the comets and random rocks that fly through space pounding planets, the instability they bring to an environment make it very difficult for life to get started.
How do we survive it? An asteroid belt. Jupiter, with it's massive gravity, was a little too small to ignite and become a small star itself. Instead, it orbits Sol, our sun. About halfway between Jupiter and the sun is an asteriod belt, held there by the gravity from both. This acts as a protective shell, like an egg, and asteriods that barrel toward the center of our solar system smash into this and become trapped(for the most part).
example in gif format Frankly though, there is still a lot to know about our solar system, we certainly don't have all of the information yet.
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