It's from an episode where Dexter figures out how to travel to a paradox dimension, where his mom and dad become his kids, Dee Dee becomes his wife, and Mandark becomes his best friend. Dexter grows very content with this living situation, and even comes to recognize the alternate reality as his home. However, the major conflict in the story is that Dexter cannot survive more than 24 hours in the alternate reality, so he must leave them at the end of the episode. It's incredibly emotional, and the ultimate source of "Coming Home."