The universe is huge, spanning in all directions for trillions of miles. Life managed to appear on this planet by chance. The probability of life appearing other places in the universe it so high, it's not really possible to simply 'believe' in it. Life can evolve almost anywhere. Humanity is just a grain of sand out of the entire beach.
There is a good chance that there is intelligent alien life out there somewhere. The universe is such a huge place that there is bound to me some kind of intelligent life out there. We have just not discovered it yet, and maybe we never will but it doesn't mean it isn't out there.