He's the only person who just rolls with everything and seems to appreciate how absurd the situation is. His line, "We're not going to Guam, are we?" basically sums up who he is. He didn't freak out, or try to find out what this means or how it affects his feelings. He's just like, "Well, let's not try to crash the plane then." Plus, he just doesn't seem impressed all that much with what's going on. Can you imagine any of the characters using their typical crap to manipulate him?

Ben would be all "you see, Frank, there's this box. It's a magic box and anything you want will come out of the box. Do you want to see the b-" FRANK LAPIDUS IS NOT IMPRESSED WITH YOUR BULLSHIT

John would pull his mystical crap about "this is a place of miracles" and how he's "seen into the island's so-"

FRANK LAPIDUS IS NOT IMPRESSED WITH YOUR BULLSHIT

Jack would be all hopped-up on goof pills, going on about, "Ahhh, I'm so unhappy, my daddy hates me, we've got to go back to the island, check out my beard I'm so desponde-"

FRANK LAPIDUS IS NOT IMPRESSED WITH YOUR BULLSHIT

It makes you wonder what kind of shit this man has seen that the island seems nothing more than an irritating trip to a mediocre theme park with his brother's kids.