It's rude to cut out on someone with no explanation.
Frank is at a party with his friend when he starts getting tired.
Frank wants to go home.
Frank takes a minute to inform his friend that he's leaving before heading home.
Frank leaves the party immediately without telling his friend he's going.
Frank's friend enjoys himself at the party without having to worry about Frank's whereabouts.
Frank's friend spends a lot of time looking for Frank and worries about what might have happened to him.