Proof is not always needed, otherwise theory would not exist and be acted on. Many proofs were resolved from theory.
In theory, intelligent life could exist on other worlds. Likewise, in theory, the opposite could exist. Either statement is probable. The difference lies in;
"No way in hell life could exist outside of our world".
versus
"While possible, I think it highly improbable life, as we know it, could exist on another planet".
Conceptually, they both mean the same thing, but the latter makes an attempt to honor the probabilites of life. The first statement is based on ignorance, or arrogance, or both and speaks from an emotional viewpoint.