Kitty said that. Kitty was correct about that, but, "How do we know nothing can come back through? What if we just haven't found anything that could yet?" His counter argument was more devil's advocate than anything else, but no one here actually understood the doors exactly, and how they truly worked. Hell no one could pin down the exact way that they functioned.
Which actually concerned John more, that some strange higher power was the reason behind this nonsense. That they would be stuck dealing with whatever came their way, and had zero control over their own lives really. The way people came and gone. It was all unsettling. But Kitty's fingers touching him, really was something soothing.
Of course, Kitty was able to get her own light laugh from John. "You never know. There was eight of them. One of them had to cop a feel. I mean how could you not? Look at me." As if there was any question on the why. "I'm fucking beautiful." He teased, finally shifting his weight to lean more on Kitty than sit next to.
The doors were unpleasant at best for John. It seemed nine times out of ten, he would be left with hating his decision on going through, and this last time was no exception. Just as Kitty was saying, based on the things he had learned, it was like the doors were purposely stuffing them in horrible situations. John hated it.