You shouldn't enter someone's apartment without their permission.
Jason's neighbor keeps their apartment unlocked while they're in, and they have a nice music collection.
Jason wants to have a look at his neighbor's music collection.
Jason knocks on his neighbor's door and asks if he can come in to check out their music, and they say sure.
Jason let's himself in to his neighbors apartment and looks at their music collection.
Jason's neighbor feels comfortable with inviting Jason into their apartment to look at their music collection.
When his neighbor sees Jason in their apartment they are freaked out about him coming in without asking.