Why are people obsessed? Easy. So many questions that don't make sense:
1) A guy goes on a trip with his fiancé and returns without her, says nothing, and lawyers up before the police become involved. What did he tell the parents? When did Petito's family find out Laundrie was back? What were the discussions like?
2) The police begin to investigate and they seize the van at Laundrie's place. He disappears a few days later. Did he run away? If so, are his parents in on it, and trying to keep the police of his scent? Did someone following the story go vigilante on Laundrie and kill or kidnap him?
3) Obviously, a lot happened between the 8/12 domestic incident and the 8/30 fake text message from Petito's phone about there not being service. What exactly though? That they had a prior incident and there is video of their dispute for everyone to see makes the people more relatable and intriguing.
4) Mental health seems to be at the forefront of this situation. Petito apparently cried for the duration of the police stop (over one hour) and couldn't regain composure. Contrast that with her being a pretty, young, and blond girl and there's a mismatch in most people's minds. Laundrie had this, "Aww shucks" kinda vibe about him the whole time, as if he were the nicest guy in the world. But obviously there's more to his story too. Figuring out the two peoples' characters also creates interest. What were these two young adults really like?