If you are already sure there is extraterrestrial life, then you already know there is extraterrestrial life.

So the question "do you want to know if there is extraterrestrial life?" really becomes a question about whether you are happy with your current certainty.

Like if someone asked me "do you really want to know the answer to 1+2?", logically I should say yes, because though I already have this knowledge, losing the knowledge would be detrimental to my life.