I think people want to believe that there are Aliens. By that I mean intelligent life not of this earth.
I would say the likelihood is that somewhere out there, there will be. It's highly unlikely they have visited us and will visit us. But never say never.
Space is just too large to comprehend, millions and millions of planets. If we've developed to how we are now then i'm sure it's likely that there are other planets that have developed further and sooner.
We probably won't ever know. Would love to know before I die though.
For the nay sayers, why would you think that there isn't any form of other life out there? And why the need to berate people who do believe and those may have seen something?