Did you ever see the movie, Little Monsters? [I Enjoy Movies]

I have, and I just got done watching it on Netflix. Such a good movie.

So either after a movie or during, if I see something I don't understand I go into this, analyze mode, to try to figure out what it is I just saw, to understand it. So if you've seen this movie then you know Brian and Maurice become friends during, but at the end, they have to say goodbye to each other.

Why is this? It's the only part of the whole movie I never understood. Let's say that when Brian gets back home from California, that underworld is still going to be there. Is it because Brian knows if he keeps going to the underworld he will turn into a monster too, and so decides to say goodbye? Can't Maurice keep coming up from the underworld to stay friends with Brian?

If you understood this ending please explain it to me lol. I honestly don't get it.

Thanks if you can help out :)