It's criminal to enter someone's house without permission.
Jack hears strange noises coming from his neighbor's house, and he grows concerned about their safety.
Jack wants to check on his neighbors.
Jack walks to his neighbor's front door and knocks, asking if everything is alright inside when they answer.
Jack checks around their house and finds that the back door is unlocked, so he walks in and looks around.
His neighbors assure him that everything is fine and appreciate that he was looking out for them and Jack feels that he has done something good for his community.
The neighbors were doing something very personal and get extremely upset that Jack walked in their house, ultimately calling the police to lodge a complaint about the incident.