I think that the universe is much too vast for us to be the only sentient beings in it.
I also think that the majority of UFO encounters can be explained away as completely mundane. People mistake all sorts of things for UFOs - planes, birds, sometimes even the moon.
Even if there were other aliens you know how hard it would be for any of them to spot us? Our electromagnetic signature is tiny compared to the sun. So tiny that you (nor I) can really comprehend how tiny it is.
