Even if there isn't definite proof of something, when the odds of that 'something' are overwhelming, most people would believe it rather than deny it. In this case when there are trillions upon trillions of stars, planets and the like, and life started on this little remote spot out of nothingness for reasons we can't pinpoint, it seems far fetched and self promoting to believe that the some form of life couldn't happen elsewhere, even in our own little galaxy which has somewhat more than a mere 100 billion stars. Just remember that "convinced" is usually a statement of opinion rather than an of absolute truth.
Moss Road brings up another good point. We judge "intelligence" as compared to our own point of view. Many animals have intelligences that we don't fully recognize, appreciate, or understand. Aliens might judge us to be intellectually lacking, while we judge them to be rather dim witted too, when neither is truly the case. We would likely have a failure to communicate.
