It is rude to leave others out of a celebration.
John has just been promoted at work and invited a group of friends over to celebrate.
John wants to have a good time.
John sees that his roommate is home and invites him to join in the celebration.
John does not invite his roommate to join the celebration, even though he is home.
John's roommate is happy to join the fun and meet John's other friends.
John's roommate feels left out and annoyed with John for hosting a party without him.