I believe that microbial life exists elsewhere in our galaxy.
I am not even saying "elsewhere in the universe." If the proposition I believe to be true is to be proved true within a generation or two, I had better limit it to our own galaxy. I will bet on its truth there.
I believe in the existence of life elsewhere because chemistry seems to be so life-striving and because life, once created, propagates itself in every possible direction. Earth's history suggests that chemicals get busy and create life given any old mix of substances that includes a bit of water, and given practically any old source of energy; further, that life, once created, spreads into every nook and cranny over a wide range of temperature, acidity, pressure, light level, and so on.
Believing in the existence of intelligent life elsewhere in the galaxy is another matter. Good luck to the SETI people and applause for their efforts, but consider that microbes have inhabited Earth for at least 75 percent of its history, whereas intelligent life has been around for but the blink of an eye, perhaps 0.02 percent of Earth's history (and for nearly all of that time without the ability to communicate into space). Perhaps intelligent life will have staying power. We don't know. But we do know that microbial life has staying power.
Now to a supposition: that Mars will be found to have harbored life and harbors life no more. If this proves to be the case, it will be an extraordinarily sobering discovery for humankind, even more so than the view of our fragile blue ball from the Moon, even more so than our removal from the center of the universe by Copernicus, Galileo, and Newton—perhaps even more so than the discovery of life elsewhere in the galaxy.