Losing a pet will break your heart, and you’re bound to lose them all. And people? People leave. They always have.
-- Martin (The Summer People (1))

Importance: Martin says this to George when Martin tells him that his father does not want him to talk to people. Martin's father forbids him from becoming friends with any of the so-called Summer People that show up on the island every year. At this point, it does not seem to make sense why his father would be so adamantly against Martin forming connections with other people. However, we later learn that the reason he is crafting his mysterious machine in the first place is to bring Martin's mother back, as she disappeared abruptly immediately after giving birth to Martin. This explains why Martin's father is so afraid of people leaving.

Call it coincidence, call it fate. This is the place you come. There’s nowhere else. There’s...
-- Darla (chapter 4)