It would be foolish to think that planet Earth is the only planet in the Universe that has any type of intelligent life. I think that the possibility of other planets having life is high. There is no doubt that mankind will eventually make contact with another lifeform. I don't think I will be alive to see it.
To assume that they didn't, would be almost impossible to believe in itself. We are on a planet, inside of a solar system, that is inside a galaxy, which harbors billions of solar systems. And on top of this, our galaxy is 1 of hundreds of billions of galaxy's. It would be almost impossible for another planet not to harbor life.
I think it would be really self centered of humans to think that they're the only life in the universe. There are probably many other planets that have life. The question is, would it be life in an intelligent form -- or that we would recognize as such? I guess we won't know until the UFO's land.
It is mathematically impossible that other planets do not harbor life. With the kepler telescope alone we have discovered thousands of stars with earth like planets orbiting within the habitable zone of said stars. There are over 100 billion galaxies and each has 100 billion stars, conservatively. To think there isn't life would be silly.
No, I do not believe that other planets harbor life, because there is no evidence that life is sustainable on other planets. There is not oxygen, carbon dioxide, and water on these planets, and those are things that other planets need to have if life is going to be sustainable on these planets.