People who doesn't believe in alien existence are in these 2 cases:
1)Fear them
2)Too much selfesteem.
Come on guys we said, existance of aliens, not that they are invading us.
I will just ask this question:
Prove me that we are the only humanoids in this galaxy.
No alien stuff, just prove me we are the only smart intelligence that is been able to build computer and internet.
That's kind of unfair. You're asking them to prove a negative, that aliens don't exist. I don't believe pink unicorns exist. Do you want me to prove that too?