The response makes him remember that it is not normal to do that. The majority of humans on his planet don't even know that supernatural creatures exist, let alone other universes. He's supposed to be much better on top of seeming normal, but as per usual, Jack doesn't always think through his words. He wants to make friends, so he tries to connect. Foolishly, at times.

"Well. It doesn't happen often!" In his case, he is the one who caused the rift, and that definitely is rare and shouldn't happen. Whoops. He hasn't considered trying to do it here, bad idea. "It's okay, the other world I went to was terrible, this one is much better. We get cars and there's no one trying to kill us yet. Not that it's normal when people do." He stumbles a little over the words at the end there.

"Maybe I'm thinking of a movie!" There. That could make sense.