The Universe is really big

The probability that were alone is so low that it would be stupid to think we are.
There are billions of galaxies and trillions of stars, which could mean an unthinkable amount of planets.
Even in the Milky-Way galaxy, there is probably life.
Therefore, were probably not alone. One day or another, we will get our proof.

Then again, it's possible we are alone, in terms of intelligent life.