Not every fish would travel aross the ocean, braving sharks, scuba divers, jellyfish, and a dentist's office, for someone they just met. But from the jump, Dory is there to support and help Marlin on his search for his son Nemo. While he doesn't always appreciate Dory at first, by the end of the movie, he recognizes her as family.
And in the sequel, "Finding Dory," both Marlin and Nemo don't hesitate to help her find her family, and then rescue her from the aquarium. We love to see it.